Redox Flow Battery Electrolytes

ABSTRACT

The present invention relates to novel combinations of redox active compounds for use as redox flow battery electrolytes. The invention further provides kits comprising these combinations, redox flow batteries, and method using the combinations, kits and redox flow batteries of the invention.

In recent years, concerns resulting from environmental consequences ofexploiting fossil fuels as the main energy sources have led to anincreasing prominence of renewable-energy systems (e.g., solar- andwind-based systems). The intermittent nature of such renewable energysources however makes it difficult to fully integrate these energysources into electrical power grids and distribution networks. Asolution to this problem are large-scale electrical energy storage (EES)systems, which are also vital for the smart grid and distributed powergeneration development. Another important application of EES iselectrification of on-ground transportation, as the replacement oftraditional combustion engines with hybrid, plug-in hybrid, and pureelectric vehicles (EVs) allows for reduction of carbon emissions andfuel savings (Soloveichik G. L. Chem. Rev. 2015, 115, 11533-11558).

The U.S. Department of Energy has identified four major challenges tothe widespread implementation of EES: cost, reliability and safety,equitable regulatory environments, and industry acceptance. Thedevelopment of novel EES technologies capable of resolving thesechallenges is critical (Soloveichik G. L. Chem. Rev. 2015, 115,11533-11558). Redox-flow batteries (RFBs)-first developed by NASA duringthe energy crisis of the 1970's and currently entering a period ofrenaissance—are among the most promising scalable EES technologies. RFBsare electrochemical systems that can repeatedly store and convertelectrical energy to chemical energy and vice versa when needed. Redoxreactions are employed to store energy in the form of a chemicalpotential in liquid electrolyte solutions which flow through a batteryof electrochemical cells during charge and discharge. The storedelectrochemical energy can be converted to electrical energy upondischarge with concomitant reversal of the opposite redox reactions.

RFBs usually include a positive electrode and a negative electrode inseparated cells and separated by an ion-exchange membrane, and twocirculating electrolyte solutions, positive and negative electrolyteflow streams, generally referred to as the “posilyte” and “negolyte”,respectively. Energy conversion between electrical energy and chemicalpotential occurs instantly at the electrodes, once the electrolytesolutions begin to flow through the cell. During discharge, electronsare released via an oxidation reaction from a high chemical potentialstate on the anode of the battery and subsequently move through anexternal circuit. Finally, the electrons are accepted via a reductionreaction at a lower chemical potential state on the cathode of thebattery. Redox-flow batteries can be recharged by inversing the flow ofthe redox fluids and applying current to the electrochemical reactor.

The capacity and energy of redox flow batteries is determined by thetotal amount of redox active species for a set system available in thevolume of electrolyte solution, whereas their current (power) depends onthe number of atoms or molecules of the active chemical species that arereacted within the redox flow battery cell as a function of time.Redox-flow batteries thus have the advantage that their capacity(energy) and their current (power) can be readily separated, andtherefore readily up-scaled. Thus, capacity (energy) can be increased byincreasing the number or size of the electrolyte tanks whereas thecurrent (power) is controlled by controlling the number and size of thecurrent collectors. Since energy and power of RFB systems areindependent variables, RFBs are inherently well suitable for largeapplications, since they scale-up in a more cost-effective manner thanother batteries. Moreover, RFBs provide a unique design flexibility asthe required capacities for any application can be provided usingtailor-made energy and power modules.

A well-established example of an RFB is the vanadium redox flow battery,which contains redox couples exclusively based on vanadium cations.Nevertheless, there is also a wide range of less commonly used inorganicflow cell chemistries, including the polysulfide-bromide battery (PSB).The wide-scale utilization of RFBs using inorganic redox materials ispresently still limited by availability and costs of the redoxmaterials. That holds even more so, whenever the redox materials arebased on redox-active transition metals such as vanadium, and/or requireprecious-metal electrocatalysts. Toxicity and associated health andenvironmental risks of inorganic redox materials (such as vanadium saltsor bromine) further limits applicability of inorganic RFBs for energystorage. That holds in particular when applying distributed, modularenergy generation technologies that use (intermittent) “green power”,such as wind, photovoltaic, or hydroelectric power. Also, theincorporated materials may constitute overheating, fire or explosionrisks.

In view of the disadvantages of RFBs based on inorganic redox species,RFBs were envisaged with different organic compounds. Novel organicredox active species for large-scale use in redox flow batteries shouldpreferably be inexpensive, with high solubility and redox potential, andexhibit fast electrode kinetics. In early 2014, Huskinson et al.developed a metal-free flow battery based on9,10-anthraquinone-2,7-disulphonic acid (AQDS) (Huskinson et al. Nature2014, 505, 195-198 and WO 2014/052682 A2). Yang et al. reported on anorganic redox flow battery with 1,2-benzoquinone-3,5-disulfonic acid(BQDS) as the catholyte, while AQDS or anthraquinone-2-sulfonic acid(AQS) was used as the anolyte (Yang et al. J. Electrochem. Soc. 2014,161, A1371-A1380). However, sheer volume of needed energy storagedemands millions of tons of active materials. To date, only a smallernumber of organic chemicals are produced worldwide at such a scale(e.g., methanol, acetic acid, and phenol). Based on scale andavailability, the “ideal” redox flow battery for large-scale deploymentshould be aqueous and use highly soluble multi-electron (i.e. highlyenergy dense) redox active species that are readily available andinexpensive as electrolytes. Derivatized anthra- and benzoquinonessuggested as electrolytes by Huskinson et al. and Yang et al. arecommercially available; however, costly and elaborate manufacture of anyof them severely limits their broad-range, large-scale employment.

In summary, despite recent advantages in the development of rechargeablebatteries, a long-felt need exists for safe, inexpensive, easy-to-use,reliable and efficient technologies for energy storage that enablesdiversification of energy supply and optimization of the energy grid,including increased penetration and utilization of renewable energies.By their unique ability to decouple power and capacity functions, redoxflow batteries are at least in principle well suited for large scaleenergy storage applications. However, development efforts have not yetachieved large-scale employment of RFBs.

Moreover, existing redox flow batteries suffer from the reliance onbattery chemistries that result in high costs of active materials andsystem engineering, low cell and system performance (e.g. round tripenergy efficiency), poor cycle life and toxicity. Thus, there remains aneed for novel electroactive redox materials, which are readilyavailable at low cost and exhibit reduced toxicity. Preferably, suchelectrolytes further provide for a high energy density, a high operatingpotential, increased cell output voltage and extended lifetime.Accordingly, there is a need in the art for improved redox flow batterychemistries and systems.

It is the object of the present invention to comply with the aboveneeds.

Although the present invention is described in detail below, it is to beunderstood that this invention is not limited to the particularmethodologies, protocols and reagents described herein as these mayvary. It is also to be understood that the terminology used herein isnot intended to limit the scope of the present invention which will belimited only by the appended claims. Unless defined otherwise, alltechnical and scientific terms used herein have the same meanings ascommonly understood by one of ordinary skill in the art.

In the following, the features of the present invention will bedescribed. These features are described for specific embodiments. Itshould, however, be understood that they may be combined in any mannerand in any number to generate additional embodiments. The variouslydescribed examples and preferred embodiments should not be construed tolimit the present invention to only explicitly described embodiments.This present description should be understood to support and encompassembodiments, which combine the explicitly described embodiments with anynumber of the disclosed and/or preferred features. Furthermore, anypermutations and combinations of all described features in thisapplication shall be considered supported by the description of thepresent application, unless it is understood otherwise.

Throughout this specification and the claims which follow, unless thecontext requires otherwise, the term “comprise”, and variations such as“comprises” and “comprising”, will be understood to imply the inclusionof a stated member, integer or step but not the exclusion of any othernon-stated member, integer or step. The term “consist of” is aparticular embodiment of the term “comprise”, wherein any othernon-stated member, integer or step is excluded. In the context of thepresent invention, the term “comprise” encompasses the term “consistof”. The term “comprising” thus encompasses “including” as well as“consisting” e.g., a composition “comprising” X may consist exclusivelyof X or may include something additional e.g., X+Y.

The terms “a” and “an” and “the” and similar reference used in thecontext of describing the invention (especially in the context of theclaims) are to be construed to cover both the singular and the plural,unless otherwise indicated herein or clearly contradicted by context.Recitation of ranges of values herein is merely intended to serve as ashorthand method of referring individually to each separate valuefalling within the range. Unless otherwise indicated herein, eachindividual value is incorporated into the specification as if it wereindividually recited herein. No language in the specification should beconstrued as indicating any non-claimed element essential to thepractice of the invention.

The word “substantially” does not exclude “completely” e.g., acomposition which is “substantially free” from Y may be completely freefrom Y. Where necessary, the word “substantially” may be omitted fromthe definition of the invention.

The term “about” in relation to a numerical value x means x±10%.

The term “alkyl” refers to the radical of saturated hydrocarbon groups,or a group derived therefrom, including linear (i.e. straight-chain)alkyl groups, branched-chain alkyl groups, cyclo-alkyl (alicyclic)groups, alkyl-substituted cyclo-alkyl groups, andcyclo-alkyl-substituted alkyl groups.

Preferably, an alkyl group contains less than 30 carbon atoms, morepreferably from 1 to 10 carbon atoms (“C₁₋₁₀ alkyl”), from 1 to 9 carbonatoms (“C₁₋₉ alkyl”), from 1 to 8 carbon atoms (“C₁₋₈ alkyl”), from 1 to7 carbon atoms (“C₁₋₇ alkyl”), or from 1 to 6 carbon atoms (“C₁₋₂alkyl”). In some embodiments, an alkyl group has 1 to 5 carbon atoms(“C₁₋₅ alkyl”). In some embodiments, an alkyl group may contain 1 to 4carbon atoms (“C₁₋₄ alkyl”), from 1 to 3 carbon atoms (“C₁₋₃ alkyl”), orfrom 1 to 2 carbon atoms (“C₁₋₂ alkyl”).

Examples of C₁₋₆ alkyl groups include methyl (C₁), ethyl (C₂), propyl(C₃) (e.g., n-propyl, isopropyl), butyl (C₄) (e.g., n-butyl, tert-butyl,sec-butyl, iso-butyl), pentyl (C₅) (e.g., n-pentyl, 3-pentanyl, amyl,neopentyl, 3-methyl-2-butanyl, tertiary amyl), and hexyl (C₆) (e.g.,n-hexyl). Additional examples of alkyl groups include n-heptyl (C₇),n-octyl (Ca), and the like.

Unless otherwise specified, each instance of an alkyl group isindependently unsubstituted (an “unsubstituted alkyl”) or substituted (a“substituted alkyl”) with one or more substituents (e.g., halogen, suchas F).

In general, the term “substituted” means that at least one hydrogenpresent on a group is replaced with a permissible substituent, e.g., asubstituent which upon substitution results in a stable compound, e.g.,a compound which does not spontaneously undergo transformation such asby rearrangement, cyclization, elimination, or other reaction. Unlessotherwise indicated, a “substituted” group has a substituent at one ormore substitutable positions of the group, and when more than oneposition in any given structure is substituted, the substituent iseither the same or different at each position. The term “substituted” iscontemplated to include substitution with all permissible substituentsof organic compounds, and includes any of the substituents describedherein that results in the formation of a stable compound. Compoundsdescribed herein contemplates any and all such combinations in order toarrive at a stable compound. Heteroatoms such as nitrogen may havehydrogen substituents and/or any suitable substituent as describedherein which satisfy the valencies of the heteroatoms and results in theformation of a stable moiety. Compounds described herein are notintended to be limited in any manner by the exemplary substituentsdescribed herein.

In certain embodiments, the alkyl group is an unsubstituted C₁₋₁₀ alkyl(such as unsubstituted C₁₋₆ alkyl, e.g., —CH₃ (Me), unsubstituted ethyl(Et), unsubstituted propyl (Pr, e.g., unsubstituted n-propyl (n-Pr),unsubstituted isopropyl (i-Pr)), unsubstituted butyl (Bu, e.g.,unsubstituted n-butyl (n-Bu), unsubstituted tert-butyl (tert-Bu ort-Bu), unsubstituted sec-butyl (sec-Bu), unsubstituted isobutyl (i-Bu)).In certain embodiments, the alkyl group is a substituted C₁₋₁₀ alkyl(such as substituted C₁₋₆ alkyl, e.g., —CF₃, Bn).

Exemplary substituents may include, for example, a halogen, a hydroxyl,a carbonyl (such as a carboxyl, an alkoxycarbonyl, a formyl, or anacyl), a thiocarbonyl (such as a thioester, a thioacetate, or athioformate), an alkoxyl, a phosphoryl, a phosphate, a phosphonate, aphosphinate, an amino, an amido, an amidine, an imine, a cyano, a nitro,an azido, a sulfhydryl, an alkylthio, a sulfate, a sulfonate, asulfamoyl, a sulfonamido, a sulfonyl, a heterocyclyl, an aralkyl, or anaromatic or heteroaromatic moiety.

Substituents may themselves be substituted. For instance, thesubstituents of a “substituted alkyl” may include both substituted andunsubstituted forms of amino, azido, imino, amido, phosphoryl (includingphosphonate and phosphinate), sulfonyl (including sulfate, sulfonamido,sulfamoyl and sulfonate), and silyl groups, as well as ethers,alkylthios, carbonyls (including ketones, aldehydes, carboxylates, andesters), —CF₃, —CN and the like. Cycloalkyls may be further substitutedwith alkyls, alkenyls, alkoxys, alkylthios, aminoalkyls,carbonyl-substituted alkyls, —CF₃, —CN, and the like.

The term “haloalkyl” refers a substituted alkyl group, wherein one ormore of the hydrogen atoms are independently replaced by a halogen,e.g., fluoro, bromo, chloro, or iodo. “Perhaloalkyl” is a subset ofhaloalkyl, and refers to an alkyl group wherein all of the hydrogenatoms are independently replaced by a halogen, e.g., fluoro, bromo,chloro, or iodo. Examples of haloalkyl groups include —CF₃, —CF₂CF₃,—CF₂CF₂CF₃, —CCl₃, —CFCl₂, —CF₂Cl, and the like.

The term “heteroalkyl” refers to an alkyl group as defined herein, whichfurther includes at least one heteroatom (e.g., 1, 2, 3, or 4heteroatoms) selected from oxygen, nitrogen, or sulfur within (i.e.,inserted between adjacent carbon atoms of) and/or placed at one or moreterminal position(s) of the parent hydrocarbon chain. Unless otherwisespecified, each instance of a heteroalkyl group is independentlyunsubstituted (an “unsubstituted heteroalkyl”) or substituted (a“substituted heteroalkyl”) with one or more substituents as definedherein.

The term “carbocyclyl” or “carbocyclic” refers to a radical of anon-aromatic cyclic hydrocarbon group having from 3 to 14 ring carbonatoms (“C₃₋₁₄ carbocyclyl”) and zero heteroatoms in the non-aromaticring system, or a group derived therefrom. Exemplary C₃₋₆ carbocyclylgroups include, without limitation, cyclopropyl (C₃), cyclopropenyl(C₃), cyclobutyl (C₄), cyclobutenyl (C₄), cyclopentyl (C₅),cyclopentenyl (C₅), cyclohexyl (C₆), cyclohexenyl (C₆), cyclohexadienyl(C₆), and the like. As the foregoing examples illustrate, in certainembodiments, the carbocyclyl group is either monocyclic (“monocycliccarbocyclyl”) or polycyclic (e.g., containing a fused, bridged or spiroring system such as a bicyclic system (“bicyclic carbocyclyl”) ortricyclic system (“tricyclic carbocyclyl”)) and can be saturated or cancontain one or more carbon-carbon double or triple bonds. “Carbocyclyl”also includes ring systems wherein the carbocyclyl ring, as definedabove, is fused with one or more aryl or heteroaryl groups wherein thepoint of attachment is on the carbocyclyl ring, and in such instances,the number of carbons continue to designate the number of carbons in thecarbocyclic ring system. Unless otherwise specified, each instance of acarbocyclyl group is independently unsubstituted (an “unsubstitutedcarbocyclyl”) or substituted (a “substituted carbocyclyl”) with one ormore substituents as defined herein.

The term “heterocyclyl” or “heterocyclic” refers to a radical of a 3- to14-membered non-aromatic ring system having ring carbon atoms and 1 to 4ring heteroatoms, wherein each heteroatom is independently selected fromnitrogen, oxygen, and sulfur (“3-14 membered heterocyclyl”), or a groupderived therefrom. In heterocyclyl groups that contain one or morenitrogen atoms, the point of attachment may be a carbon or nitrogenatom, as valency permits. A heterocyclyl group can either be monocyclic(“monocyclic heterocyclyl”) or polycyclic (e.g., a fused, bridged orspiro ring system such as a bicyclic system (“bicyclic heterocyclyl”) ortricyclic system (“tricyclic heterocyclyl”)), and may be saturated ormay contain one or more carbon-carbon double or triple bonds.Heterocyclyl polycyclic ring systems may include one or more heteroatomsin one or both rings. “Heterocyclyl” also includes ring systems whereinthe heterocyclyl ring, as defined above, is fused with one or morecarbocyclyl groups wherein the point of attachment is either on thecarbocyclyl or heterocyclyl ring, or ring systems wherein theheterocyclyl ring, as defined above, is fused with one or more aryl orheteroaryl groups, wherein the point of attachment is on theheterocyclyl ring, and in such instances, the number of ring memberscontinue to designate the number of ring members in the heterocyclylring system. Unless otherwise specified, each instance of heterocyclylis independently unsubstituted (an “unsubstituted heterocyclyl”) orsubstituted (a “substituted heterocyclyl”) with one or more substituentsas defined herein.

Exemplary 3-membered heterocyclyl groups containing 1 heteroatominclude, without limitation, azirdinyl, oxiranyl, and thiiranyl.Exemplary 4-membered heterocyclyl groups containing 1 heteroatominclude, without limitation, azetidinyl, oxetanyl, and thietanyl.Exemplary 5-membered heterocyclyl groups containing 1 heteroatominclude, without limitation, tetrahydrofuranyl, dihydrofuranyl,tetrahydrothiophenyl, dihydrothiophenyl, pyrrolidinyl, dihydropyrrolyl,and pyrrolyl-2,5-dione. Exemplary 5-membered heterocyclyl groupscontaining 2 heteroatoms include, without limitation, dioxolanyl,oxathiolanyl and dithiolanyl. Exemplary 5-membered heterocyclyl groupscontaining 3 heteroatoms include, without limitation, triazolinyl,oxadiazolinyl, and thiadiazolinyl. Exemplary 6-membered heterocyclylgroups containing 1 heteroatom include, without limitation, piperidinyl,tetrahydropyranyl, dihydropyridinyl, and thianyl. Exemplary 6-memberedheterocyclyl groups containing 2 heteroatoms include, withoutlimitation, piperazinyl, morpholinyl, dithianyl, and dioxanyl. Exemplary6-membered heterocyclyl groups containing 2 heteroatoms include, withoutlimitation, triazinanyl. Exemplary 7-membered heterocyclyl groupscontaining 1 heteroatom include, without limitation, azepanyl, oxepanyland thiepanyl. Exemplary 8-membered heterocyclyl groups containing 1heteroatom include, without limitation, azocanyl, oxecanyl andthiocanyl. Exemplary bicyclic heterocyclyl groups include, withoutlimitation, indolinyl, isoindolinyl, dihydrobenzofuranyl,dihydrobenzothienyl, tetrahydrobenzothienyl, tetrahydrobenzofuranyl,tetrahydroindolyl, tetrahydroquinolinyl, tetrahydroisoquinolinyl,decahydroquinolinyl, decahydroisoquinolinyl, octahydrochromenyl,octahydroisochromenyl, decahydronaphthyridinyl,decahydro-1,8-naphthyridinyl, octahydropyrrolo[3,2-b]pyrrole, indolinyl,phthalimidyl, naphthalimidyl, chromanyl, chromenyl,1H-benzo[e][1,4]diazepinyl, 1,4,5,7-tetrahydropyrano[3,4-b]pyrrolyl,5,6-dihydro-4H-furo[3,2-b]pyrrolyl, 6,7-dihydro-5H-furo[3,2-b]pyranyl,5,7-dihydro-4H-thieno[2,3-c]pyranyl,2,3-dihydro-1H-pyrrolo[2,3-b]pyridinyl, 2,3-dihydrofuro[2,3-b]pyridinyl,4,5,6,7-tetrahydro-1H-pyrrolo[2,3-b]pyridinyl,4,5,6,7-tetrahydrofuro[3,2-c]pyridinyl,4,5,6,7-tetrahydrothieno[3,2-b]pyridinyl,1,2,3,4-tetrahydro-1,6-naphthyridinyl, and the like.

The term “aryl” refers to a radical of a monocyclic or polycyclic (e.g.,bicyclic or tricyclic) 4n+2 aromatic ring system (e.g., having 6, 10, or14π electrons shared in a cyclic array) having 6-14 ring carbon atomsand zero heteroatoms provided in the aromatic ring system (“C₁₄ aryl”),or a group derived therefrom. In some embodiments, an aryl group has 6ring carbon atoms (“C₆ aryl”; e.g., phenyl). In some embodiments, anaryl group has 10 ring carbon atoms (“C₁₀ aryl”; e.g., naphthyl such as1-naphthyl and 2-naphthyl). In some embodiments, an aryl group has 14ring carbon atoms (“C₄ aryl”; e.g., anthracyl). “Aryl” also includesring systems wherein the aryl ring, as defined above, is fused with oneor more carbocyclyl or heterocyclyl groups wherein the radical or pointof attachment is on the aryl ring, and in such instances, the number ofcarbon atoms continue to designate the number of carbon atoms in thearyl ring system. Unless otherwise specified, each instance of an arylgroup is independently unsubstituted (an “unsubstituted aryl”) orsubstituted (a “substituted aryl”) with one or more substituents asdefined herein.

The term “aryl” as used herein thus includes 5-, 6-, and 7-memberedsingle-ring aromatic groups that may include from zero to fourheteroatoms, for example, benzene, pyrrole, furan, thiophene, imidazole,oxazole, thiazole, triazole, pyrazole, pyridine, pyrazine, pyridazineand pyrimidine, and the like. Those aryl groups having heteroatoms inthe ring structure may also be referred to as “heteroaryls”, “arylheterocycles” or “heteroaromatics.” The aromatic ring may be substitutedat one or more ring positions with such substituents as described above,for example, halogen, azide, alkyl, aralkyl, alkenyl, alkynyl,cycloalkyl, hydroxyl, alkoxyl, amino, nitro, sulfhydryl, imino, amido,phosphate, phosphonate, phosphinate, carbonyl, carboxyl, silyl, ether,alkylthio, sulfonyl, sulfonamido, ketone, aldehyde, ester, heterocyclyl,aromatic or heteroaromatic moieties, —CF₃, —CN, or the like. The term“aryl” also includes polycyclic ring systems having two or more cyclicrings in which two or more carbons are common to two adjoining rings(the rings are “fused rings”) wherein at least one of the rings isaromatic, e.g., the other cyclic rings can be cycloalkyls,cycloalkenyls, cycloalkynyls, aryls and/or heterocyclyls.

The term “heteroaryl” refers to a radical of a 5-14 membered monocyclicor polycyclic (e.g., bicyclic, tricyclic) 4n+2 aromatic ring system(e.g., having 6, 10, or 14n electrons shared in a cyclic array) havingring carbon atoms and 1-4 ring heteroatoms provided in the aromatic ringsystem, wherein each heteroatom is independently selected from nitrogen,oxygen, and sulfur (“5-14 membered heteroaryl”), or a group derivedtherefrom. In heteroaryl groups that contain one or more nitrogen atoms,the point of attachment may be a carbon or nitrogen atom, as valencypermits. Heteroaryl polycyclic ring systems may include one or moreheteroatoms in one or both rings. “Heteroaryl” includes ring systemswherein the heteroaryl ring, as defined above, is fused with one or morecarbocyclyl or heterocyclyl groups wherein the point of attachment is onthe heteroaryl ring, and in such instances, the number of ring memberscontinue to designate the number of ring members in the heteroaryl ringsystem. “Heteroaryl” also includes ring systems wherein the heteroarylring, as defined above, is fused with one or more aryl groups whereinthe point of attachment is either on the aryl or heteroaryl ring, and insuch instances, the number of ring members designates the number of ringmembers in the fused polycyclic (aryl/heteroaryl) ring system.Polycyclic heteroaryl groups wherein one ring does not contain aheteroatom (e.g., indolyl, quinolinyl, carbazolyl, and the like) thepoint of attachment can be on either ring, i.e., either the ring bearinga heteroatom (e.g., 2-indolyl) or the ring that does not contain aheteroatom (e.g., 5-indolyl). Unless otherwise specified, each instanceof a heteroaryl group is independently unsubstituted (an “unsubstitutedheteroaryl”) or substituted (a “substituted heteroaryl”) with one ormore substituents.

Exemplary 5-membered heteroaryl groups containing 1 heteroatom include,without limitation, pyrrolyl, furanyl, and thiophenyl. Exemplary5-membered heteroaryl groups containing 2 heteroatoms include, withoutlimitation, imidazolyl, pyrazolyl, oxazolyl, isoxazolyl, thiazolyl, andisothiazolyl. Exemplary 5-membered heteroaryl groups containing 3heteroatoms include, without limitation, triazolyl, oxadiazolyl, andthiadiazolyl. Exemplary 5-membered heteroaryl groups containing 4heteroatoms include, without limitation, tetrazolyl. Exemplary6-membered heteroaryl groups containing 1 heteroatom include, withoutlimitation, pyridinyl. Exemplary 6-membered heteroaryl groups containing2 heteroatoms include, without limitation, pyridazinyl, pyrimidinyl, andpyrazinyl. Exemplary 6-membered heteroaryl groups containing 3 or 4heteroatoms include, without limitation, triazinyl and tetrazinyl,respectively. Exemplary 7-membered heteroaryl groups containing 1heteroatom include, without limitation, azepinyl, oxepinyl, andthiepinyl. Exemplary 5,6-bicyclic heteroaryl groups include, withoutlimitation, indolyl, isoindolyl, indazolyl, benzotriazolyl,benzothiophenyl, isobenzothiophenyl, benzofuranyl, benzoisofuranyl,benzimidazolyl, benzoxazolyl, benzisoxazolyl, benzoxadiazolyl,benzthiazolyl, benzisothiazolyl, benzthiadiazolyl, indolizinyl, andpurinyl. Exemplary 6,6-bicyclic heteroaryl groups include, withoutlimitation, naphthyridinyl, pteridinyl, quinolinyl, isoquinolinyl,cinnolinyl, quinoxalinyl, phthalazinyl, and quinazolinyl. Exemplarytricyclic heteroaryl groups include, without limitation,phenanthridinyl, dibenzofuranyl, carbazolyl, acridinyl, phenothiazinyl,phenoxazinyl and phenazinyl.

The term “unsaturated bond” refers to a double or triple bond.

The term “unsaturated” or “partially unsaturated” refers to a moietythat includes at least one double or triple bond.

The term “saturated” refers to a moiety that does not contain a doubleor triple bond, i.e., the moiety only contains single bonds.

A group is optionally substituted unless expressly provided otherwise.The term “optionally substituted” refers to a group which may besubstituted or unsubstituted as defined herein.

The term “aliphatic group” refers to a straight-chain, branched-chain,or cyclic non-aromatic saturated or unsaturated hydrocarbon group andincludes as alkyl groups, alkenyl groups, and alkynyl groups.

The terms “alkenyl” and “alkynyl” refer to unsaturated aliphatic groupsanalogous in length and possible substitution to the alkyls describedabove, but that contain at least one double or triple bond respectively.

The terms “alkoxyl” or “alkoxy” as used herein refers to group offormula —OR, wherein R is an alkyl group, as defined herein. Exemplaryalkoxyl groups include methoxy, ethoxy, propyloxy, tert-butoxy and thelike.

The term “aralkyl”, as used herein, refers to an alkyl group substitutedwith an aryl group (e.g., an aromatic or heteroaromatic group).

The term “carbocycle”, as used herein, refers to an aromatic ornon-aromatic ring in which each atom of the ring is carbon.

The term “carbonyl” refers to a group which contains a carbon atomconnected with a double bond to an oxygen or a sulfur atom. Examples ofmoieties which contain a carbonyl include aldehydes, ketones, carboxylicacids, amides, esters, anhydrides, etc.

The term “ester” refers to groups or molecules which contain a carbon ora heteroatom bound to an oxygen atom which is bonded to the carbon of acarbonyl group. The term “ester” includes alkoxycarboxy groups such asmethoxycarbonyl, ethoxycarbonyl, propoxycarbonyl, butoxycarbonyl,pentoxycarbonyl, etc. The alkyl, alkenyl, or alkynyl groups are asdefined above.

The term “carbonyl” includes groups such as “alkylcarbonyl” groups wherean alkyl group is covalently bound to a carbonyl group,“alkenylcarbonyl” groups where an alkenyl group is covalently bound to acarbonyl group, “alkynylcarbonyl” groups where an alkynyl group iscovalently bound to a carbonyl group, “arylcarbonyl” groups where anaryl group is covalently attached to the carbonyl group. Furthermore,the term also refers to groups where one or more heteroatoms arecovalently bonded to the carbonyl moiety. For example, the term includesmoieties such as, for example, aminocarbonyl moieties, (where a nitrogenatom is bound to the carbon of the carbonyl group, e.g., an amide),aminocarbonyloxy moieties, where an oxygen and a nitrogen atom are bothbond to the carbon of the carbonyl group (e.g., also referred to as a“carbamate”). Furthermore, aminocarbonylamino groups are also includedas well as other combinations of carbonyl groups bound to heteroatoms(e.g., nitrogen, oxygen, sulfur, etc. as well as carbon atoms), such asthiocarbonyl, thiocarboxylic acid and thiolformate. Furthermore, theheteroatom can be further substituted with one or more alkyl, alkenyl,alkynyl, aryl, aralkyl, acyl, etc. moieties.

The term “ether” refers to groups or molecules which contain an oxygenbonded to two different carbon atoms or heteroatoms. For example, theterm includes “alkoxyalkyl” which refers to an alkyl, alkenyl, oralkynyl group covalently bonded to an oxygen atom which is covalentlybonded to another alkyl group.

The term “thioether” refers to groups or molecules which contain asulfur atom bonded to two different carbon or hetero atoms. Examples ofthioethers include, but are not limited to alkthioalkyls,alkthioalkenyls, and alkthioalkynyls. The term “alkthioalkyls” includecompounds with an alkyl, alkenyl, or alkynyl group bonded to a sulfuratom which is bonded to an alkyl group. Similarly, the term“alkthioalkenyls” and alkthioalkynyls” refer to compounds or moietieswhere an alkyl, alkenyl, or alkynyl group is bonded to a sulfur atomwhich is covalently bonded to an alkynyl group.

The term “amine” or “amino” includes compounds where a nitrogen atom iscovalently bonded to at least one carbon atom or heteroatom. The term“alkyl amino” includes groups and compounds where the nitrogen is boundto at least one additional alkyl group. The term “dialkyl amino”includes groups where the nitrogen atom is bound to at least twoadditional alkyl groups.

The term “arylamino” and “diarylamino” include groups where the nitrogenis bound to at least one or two aryl groups, respectively. The term“alkylarylamino,” “alkylaminoaryl” or “arylaminoalkyl” refers to anamino group which is bound to at least one alkyl group and at least onearyl group. The term “alkaminoalkyl” refers to an alkyl, alkenyl, oralkynyl group bound to a nitrogen atom which is also bound to an alkylgroup.

The term “amine” or “amino” in particular refers to a —NH₂ group,preferably including any of its protonation states, such as —NH₃.

The term “amide” or “aminocarboxy” includes compounds or moieties whichcontain a nitrogen atom which is bound to the carbon atom of a carbonylor a thiocarbonyl group. The term includes “alkaminocarboxy” groupswhich include alkyl, alkenyl, or alkynyl groups bound to an amino groupbound to a carboxy group. It includes arylaminocarboxy groups whichinclude aryl or heteroaryl moieties bound to an amino group which isbound to the carbon of a carbonyl or thiocarbonyl group. The terms“alkylaminocarboxy,” “alkenylaminocarboxy,” “alkynylaminocarboxy,” and“arylaminocarboxy” include moieties where alkyl, alkenyl, alkynyl andaryl moieties, respectively, are bound to a nitrogen atom which is inturn bound to the carbon of a carbonyl group.

The term “nitro” refers to a —NO₂ group.

The term “halo” or “halogen” refers to fluorine (fluoro, —F), chlorine(chloro, — Cl), bromine (bromo, —Br), or iodine (iodo, —I) groups.

The term “thiol” or “sulfhydryl” refers to a —SH group.

The term “hydroxyl” refers to a —OH group, preferably including all ofits protonation states, such as —O⁺. The term “sulfonyl” refers to a—SO₃H group, preferably including all of its protonation states, such as—SO₃ ⁻.

The term “phosphoryl” refers to a —PO₃H₂ group, preferably including allof its protonation states, such as —PO₃H⁻ and —PO₃ ²⁻.

The term “phosphonyl” refers to a —PO₃R₂ group, wherein each R is H oralkyl, provided at least one R is alkyl, as defined herein, preferablyincluding all of its protonation states, such as —PO₃R⁻.

The term “oxo” refers to a ═O group.

The term “carboxyl” refers to a —COOH group, preferably including all ofits protonation states, such as —COO⁻.

The term “oxy” refers to a —O group.

The term “quinone” refers to a class of cyclic organic compounds thatinclude fully conjugated —C(═O)— groups and carbon-carbon double bonds.In one example, the term “quinone” refers to organic compounds that areformally derived from aromatic compounds by replacement of an evennumber of —CH=groups with —C(═O)— groups with the double bondsrearranged as necessary to provide a fully conjugated cyclic dione,tetra-one, or hexa-one structure. The term inter alia covers substitutedand unsubstituted quinones derived from mono-di- and trihydroaromaticsystems comprising 1 to 3 fused carbon cyclic rings in their oxidizedand reduced forms.

The term “conjugated” when referring to two functional groups (having adouble bond) means that the two groups are part of a connected system ofp-orbital delocalized electrons with alternating single and multiplebonds. The two groups also include a degree of unsaturation. Forexample, conjugated groups may include multiple double bonds or aromaticgroups (e.g., phenyl) between the groups. Moreover, if the two groupsadjacent, the groups are also conjugated.

The term “standard electrode potential” means the electrical potential(i.e., the voltage developed) of a reversible electrode at standardstate in which solutes are at an effective concentration of 1 mol/liter,the activity for each pure solid, pure liquid, or for water (solvent) is1, the pressure of each gaseous reagent is 1 atm., and the temperatureis 25° C. Standard electrode potentials are reduction potentials.

The term “OCV” or “open circuit voltage” refers to the battery voltageunder the equilibrium conditions, i.e. the voltage when no current isflowing in or out of the battery, and, hence no reactions occur insidethe battery. The OCV can be determined from the reduction potentials ofthe half-cell reactions that occur at the positive electrode (Eca_(t))and the negative electrode (E_(an)) according to equation (i):

OCV=E _(cat) −E _(an)  Eq. (i)

The OCV is a function of the state-of-charge (SOC).

The term “current density” refers to the current per unit geometric areapassed by an electrochemical cell. The current density may be determinedby measuring the amount of current passed by an electrochemical cell anddividing by the geometric area of the electrode.

The term “current efficiency” refers to the ratio of total charge drawnduring a period of discharge to the total charge passed during acorresponding period of charge. The current efficiency can be determinedby counting the amp-hours passed while charging the redox flow batterybetween two states (e.g., 0% to 100% state of charge), and counting theamp-hours passed while discharging the battery to back to the originalstate (e.g., 100% to 0% state of charge), and dividing the amp-hours forthe discharge step by the amp-hours for the charge step.

The term “voltage efficiency” of a redox flow battery refers to theratio of the cell voltage at discharge to the voltage at charge. Voltageefficiency is determined for a given current density, for example bymeasuring the voltage at a given current density while charging anddividing by the voltage at the same current density while discharging.The voltage efficiency may be affected by a number of additionalfactors, including state of charge.

The “state-of-charge” or “SOC” of an electrolyte is determined from theconcentration of the charged form of the redox active compound(X_(charge)) and the concentration of the discharged from of the redoxactive compound (X_(discharge)) according to Eq. (ii).

$\begin{matrix}{{{SOC}\mspace{14mu} \%} = \frac{X_{charge}}{X_{charge} + X_{discharge}}} & {{Eq}.\mspace{14mu} ({ii})}\end{matrix}$

The terms “positive electrode” and “negative electrode” are defined suchthat the positive electrode is intended to operate as a more positivepotential than that of the negative electrode. The positive electrode isassociated with the positive electrolyte and the positive redox activecompound. The negative electrode is associated with the negativeelectrolyte and the negative redox active compound.

The present invention provides novel combinations of redox activequinones and hydroquinones, which are particularly useful in redox flowbattery applications.

1. Redox Flow Battery: General Principle

In its simplest form, a redox flow battery may be thought of as arechargeable battery with a continuous flow of one electrolyte past itsnegative electrode and a continuous flow of another electrolyte past itspositive electrode. The positive and negative electrolytes may bereferred to as “posilyte” and “negolyte”, respectively. The electrolytesare stored separately and cycle to and from a power-converting device,such as an electrochemical cell stack, when charging (i.e., absorbingexcess electricity from the power source) or discharging (i.e.,delivering electricity to the power source). During charge, the posilyte“P” is oxidized (i.e., loses electrons) to a higher oxidation state andthe negolyte “N” is reduced (i.e., accepts electrons) to a loweroxidation state. During discharge, when electricity is utilized from theflow battery, the current direction and thus, the reduction andoxidation reactions are reversed. Accordingly, “P” is reduced to a loweroxidation state and “N” is oxidized to a higher oxidation state. Theelectrolyte reactions are schematically illustrated in Reaction Scheme(i) and (ii) below.

$\begin{matrix}{{{P^{n}\overset{yields}{}P^{n - z}} + {{ze}^{-}({Charge})}}P^{n - z} + {{ze}^{-}\overset{yields}{}{P^{n}({Discharge})}}} & {{Reaction}\mspace{14mu} {Scheme}\mspace{14mu} (i)} \\{{N^{n - z} + {{ze}^{-}\overset{yields}{}{N^{n}({Charge})}}}{{N^{n}\overset{yields}{}N^{n - z}} + {{ze}^{-}({Discharge})}}} & {{Reaction}\mspace{14mu} {Scheme}\mspace{14mu} ({ii})}\end{matrix}$

P: posilyte; N: negolyte; e⁻: electron; n: oxidation number; z: numberof electrons

For the overall reaction to happen spontaneously, the redox potentialdifference (AV) between the anode and cathode reactions must be >0.0 V.This is because the Gibbs free energy (ΔG) of the reaction must benegative for a spontaneous reaction to occur i.e. ΔG=−nFΔE, where n isthe number of electrons per molecule and F is the Faraday constant.

In redox flow barriers, P and N are separated into chambers where thepositive and negative electrode are electronically connected in acircuit (for electron flow) and ionically connected with anion-conducting separator (e.g. a polymer membrane) which allowspositively charged ions (usually H⁺) to flow from one chamber to theother chamber to maintain electroneutrality. If the oxidation andreduction reaction are reversible, the battery can be recharged forreuse.

The inventive combination is a combination of compounds, preferablyquinone compounds, represented by General Formulas (1), (2) or (3) asdefined herein.

In a first aspect, the present invention provides a combination of afirst redox active composition comprising a first redox active compound,the first redox active compound being characterized by any of generalformulas (1)-(3), more preferably general formulas (1) or (2) and mostpreferably general formula (1), or mixtures thereof; and (2) a secondredox active composition comprising a second redox active compound, thesecond redox active compound being characterized by any of generalformulas (1)-(3), more preferably general formulas (2) or (3), and mostpreferably by general formula (3), or mixtures thereof:

wherein each of R¹-R⁴ in formula (1); R¹-R⁶ in formula (2); and/or R¹-R⁸in formula (3) is independently selected from hydrogen; hydroxyl;carboxy; optionally substituted C₁₋₆ alkyl optionally comprising atleast one heteroatom selected from N, O and S, including —C_(n)H_(2n)OH,—C_(n)H_(2n)NH₂ and —C_(n)H_(2n)SO₃H, wherein n is an integer selectedfrom 1, 2, 3, 4, 5, or 6; carboxylic acids; esters; halogen; optionallysubstituted C₁₋₆ alkoxy, including methoxy and ethoxy; optionallysubstituted amino, including primary, secondary, tertiary and quaternaryamines, in particular —NH₂/NH₃ ⁺, —NHR/NH₂R⁺, —NR₂/NHR₂ and —NR₃ ⁺,where R is H or optionally substituted C₁₋₆ alkyl optionally comprisingat least one heteroatom selected from N, O and S, including—C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, —C_(n)H_(2n)NR₂, —C_(n)H_(2n)CO₂H and—C_(n)H_(2n)SO₃H, wherein n is an integer selected from 1, 2, 3, 4, 5,or 6, where R is H or optionally substituted C₁₋₆ alkyl optionallycomprising at least one heteroatom selected from N, O and S, including—C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, C H₂.CO₂H and —C_(n)H_(2n)SO₃H; amide;nitro; carbonyl; phosphoryl; phosphonyl; cyanide; and sulfonyl (—SO₃H),wherein preferably at least one of R¹-R⁴ in formula (1); R¹-R⁶ informula (2); and/or R¹-R⁸ in formula (3) is selected from —SO₃H;optionally substituted C₁₋₆ alkyl optionally comprising at least oneheteroatom selected from N, O and S, including —C_(n)H_(2n)OH,—C_(n)H_(2n)NH₂ and —C_(n)H_(2n)SO₃H, wherein n is an integer selectedfrom 1, 2, 3, 4, 5, or 6; and optionally substituted amine, inparticular —NH₂/NH₃ ⁺, —NHR/NH₂R, —NR₂/NHR₂ ⁺ and —NR₃ ⁺, where R is Hor optionally substituted C₁₋₆ alkyl optionally comprising at least oneheteroatom selected from N, O and S, including —C_(n)H_(2n)OH,—C_(n)H_(2n)NH₂, —C_(n)H_(2n)NR₂, —C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H,wherein n is an integer selected from 1, 2, 3, 4, 5, or 6, where R is Hor optionally substituted C₁₋₆ alkyl optionally comprising at least oneheteroatom selected from N, O and S, including —C_(n)H_(2n)OH,—C_(n)H_(2n)NH₂, C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H; and optionallysubstituted C₁₋₆ alkoxy, preferably methoxy.

According to a different annotation, and without changing the scope ofthe invention, the inventive combination may be defined as follows: Acombination of a first redox active composition comprising a first redoxactive compound, the first redox active compound being characterized byany of general formulas (1)-(3), more preferably general formulas (1) or(2) and most preferably general formula (1), or mixtures thereof; and(2) a second redox active composition comprising a second redox activecompound, the second redox active compound being characterized by any ofgeneral formulas (1)-(3), more preferably general formulas (2) or (3),and most preferably by general formula (3), or mixtures thereof:

wherein R¹-R¹⁸ are each independently selected from hydrogen; hydroxyl;carboxy; optionally substituted C₁₋₆ alkyl optionally comprising atleast one heteroatom selected from N, O and S, including —C_(n)H_(2n)OH,—C_(n)H_(2n)NH₂ and —C_(n)H_(2n)SO₃H, wherein n is an integer selectedfrom 1, 2, 3, 4, 5, or 6; carboxylic acids; esters; halogen; optionallysubstituted C₁₋₆ alkoxy, including methoxy and ethoxy; optionallysubstituted amino, including primary, secondary, tertiary and quaternaryamines, in particular —NH₂/NH₃ ⁺, —NHR/NH₂R⁺, —NR₂/NHR₂ ⁺ and —NR₃ ⁺,where R is H or optionally substituted C₁6 alkyl optionally comprisingat least one heteroatom selected from N, O and S, including—C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, —C_(n)H_(2n)NR₂, —C_(n)H_(2n)CO₂H and—C_(n)H_(2n)SO₃H, wherein n is an integer selected from 1, 2, 3, 4, 5,or 6, where R is H or optionally substituted C₁₋₆ alkyl optionallycomprising at least one heteroatom selected from N, O and S, including—C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, C_(n)H_(2n)CO₂H and —CH₂SO₃H; amide;nitro; carbonyl; phosphoryl; phosphonyl; cyanide; and sulfonyl (—SO₃H),wherein preferably at least one of R¹-R⁴ in General Formula (1), atleast one of R⁵-R¹⁰ in General Formula (2) and/or at least one ofR¹¹-R¹⁸ in General Formula (3) is selected from —SO₃H; optionallysubstituted C₁₋₆ alkyl optionally comprising at least one heteroatomselected from N, O and S, including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂ and—C_(n)H_(2n)SO₃H, wherein n is an integer selected from 1, 2, 3, 4, 5,or 6; and optionally substituted amine, in particular —NH₂/NH₃ ⁺,—NHR/NH₂R⁺, —NR₂/NHR₂ ⁺ and —NR₃, where R is H or optionally substitutedC₁₋₆ alkyl optionally comprising at least one heteroatom selected fromN, O and S, including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, —C_(n)H_(2n)NR₂,—C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H, wherein n is an integer selectedfrom 1, 2, 3, 4, 5, or 6, where R is H or optionally substituted C₁₋₆alkyl optionally comprising at least one heteroatom selected from N, Oand S, including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, C_(n)H_(2n)CO₂H and—C_(n)H_(2n).O₃H; and optionally substituted C₁₋₆ alkoxy, preferablymethoxy.

Therein, “R¹-R⁴ of General Formula (1)” correspond to R¹-R⁴. “R¹-R⁶ ofGeneral Formula (2)” correspond to R⁵-R¹⁰. “R¹-R⁸ of General Formula(3)” correspond to R¹¹-R¹⁸.

2. Redox Characteristics

As used herein, the term “redox active” refers to the capability of acompound or a composition comprising said compound to participate in aredox reaction. “Redox active” materials are thus preferably capable ofundergoing a change in oxidation state when subjected to appropriatereduction or oxidation conditions, e.g. during operation of anelectrochemical system, such as a redox flow battery.

A “redox active” compound may be—under appropriate redoxconditions—“oxidized”, i.e. loses electrons, yielding an “oxidationproduct” (i.e., oxidized form) of the compound. A “redox activecompound” may be—under appropriate redox conditions—“reduced”, i.e.accepts electrons, yielding a “reduction product” (i.e., reduced form”)of the compound. A “redox active” compound may thus be understood as achemical compound, which may form a pair or couple of an oxidized andreduced form (“redox pair”, “redox couple”).

“Redox active compositions” comprise at least one “redox activecompound” typically dissolved in a suitable solvent, such as water. Someredox active compositions comprise a mixture of several redox activecompounds as disclosed herein.

It will be understood that the term “redox active compound” encompassescompounds in at least one, typically two or even more than two oxidationstates.

The “redox active compound” of the first and second redox activecomposition may thus be present both in its reduced and in its oxidizedform, i.e. forming a redox couple. Specifically, when referring to“redox active compounds according to General Formulas (1), (2) and (3)”herein, reference is made to both redox active compounds both in theiroxidized form (as represented by General Formula (1)(b), (2)(b) and(3)(b), respectively) and their reduced form (as represented by GeneralFormula (1)(a), (2)(a) and (3)(a), respectively). Preferably, the “redoxactive compounds” of the inventive combination may be classified as“quinone compounds”, which may be present in their oxidized (quinone) orreduced (hydroquinone) forms or both, forming a quinone/hydroquinoneredox couple. The term “quinone compound” is thus inclusive and refersto oxidized (quinone) and reduced (hydroquinone) forms of the samecompound.

Each type of redox active compound present in the inventive compositiontypically exhibits a specific redox potential. Generally, the redoxpotentials of the first and second redox active compound may be the sameor different. When the redox potentials are different, the redox activecompound with the higher redox potential may be referred to as a“positive redox active compound”, and in the context of redox flowbatteries, the corresponding redox active composition/electrolyte may bereferred to as the “positive electrolyte” (posilyte). Likewise, theredox active compound with the lower redox potential may be referred toas the “negative redox active compound”, and in the context of redoxflow batteries, the corresponding redox active composition/electrolytemay be referred to as the “negative electrolyte” (negolyte).

Redox active compositions may be employed as “posilytes” or “negolytes”in redox flow batteries based on their relative redox potentials.Generally, redox active compounds and compositions defined herein may inprinciple be used as either “posilyte” or “negolyte”, depending on thereduction potential of the redox active compound present in theelectrolyte of the respective counter electrode.

3. Redox Active Quinone Compounds

3.1 Quinone Compounds: Advantages

Redox active compounds of the inventive combination are preferablyclassified or classifiable as “quinone compounds”. Quinone compounds areadvantageously capable of undergoing reversible and fast electrochemicaltransformations between their oxidized (quinone) and reduced(hydroquinone) forms. Quinone/hydroquinone redox couples areparticularly suitable for redox flow battery applications, as theirrapid redox cycling characteristics enable high battery discharge andcharge rates.

The facility of electrochemical transformation is characterized by thekinetic parameter termed exchange current density. The standard rateconstant for the quinone/hydroquinone couple is of the order of 10⁻³ cms⁻¹. This value of rate constant corresponds to very fast reaction ratescomparable to other electrochemical couples such as the vanadium redoxcouple.

In aqueous solution, quinone compounds typically undergo fasttwo-electron reduction with or without proton transfer depending on pH.Under acidic conditions, quinones are thus typically reduced tohydroquinones, whereby at least one oxo-group bound to the aromatic ringof the quinone is converted into a hydroxyl-group (cf. StructuralFormulas (1)(a), (2)(a) and (3)(a).

3.2 Quinones Compounds: Reduction/Oxidation Reactions

Redox active compounds according to General Formulas (1), (2) and (3),preferably quinone compounds, may undergo oxidation or reductionaccording to reaction scheme (I), (II) or (III) depicted below. Theseoxidation or reduction reactions may involve one-electron transfers ormultiple-electron transfers. A redox active compound of the inventivecombination, preferably a quinone compound, may be oxidized or reduced,respectively, by one electron, more preferably by two electrons. Aone-electron redox reaction may result in the formation of semiquinones,i.e. intermediate free radicals generated in the conversion of quinonesto/from hydroquinones. A two-electron transfer in the conversion ofquinones to/from hydroquinones preferably yields hydroquinones/quinones,respectively. Two-electron transfers may occur simultaneously or in astepwise manner.

The equilibrium arrows in Reaction Schemes (I)-(II) are not intended toindicate a reaction mechanism of the electron and proton transfersbetween forms of the quinone compounds, but are used to indicate the netchange in the number of electrons and protons present in each compound

Quinones (i.e. quinone compounds in their oxidized form) as representedby General Formulas (1)(b), (2)(b) and (3)(b), which may generally bereferred to as “Q¹”, “Q²”, “Q³” . . . and so forth herein. Quinones maybe reduced to form hydroquinones as represented by General Formulas(1)(a), (2)(a) and (3)(a), which may generally be referred to as “H₂Q¹”,H₂Q²”, “H₂Q³” and so forth herein. Each number in superscript indicatesa different (hydro-)quinone species. Each quinone and its reducedhydroquinone counterpart forms a redox couple (“Q¹/H₂Q¹”, “Q²/H₂Q²” . .. )

Preferably, the first redox active composition of the inventivecombination may comprise a redox active compound exhibiting a higherstandard reduction potential and may be used as the positive electrodeelectrolyte. Preferably, the second redox active composition of theinventive combination comprise a redox active compound exhibiting alower reduction potential and may be used as the negative electrodeelectrolyte. Alternatively, in some applications the first redox activecomposition may be used as the negative electrode electrolyte and thesecond redox active composition may be the positive electrodeelectrolyte.

During charging of the redox flow battery, a potential difference maytypically be applied, causing the reduction of a quinone preferablyrepresented by General Formula (1)(b), yielding a hydroquinonerepresented by General Formula (1)(a) according to Reaction Scheme (I);and the oxidation of a hydroquinone preferably represented by GeneralFormula (3)(b), yielding a quinone represented by General Formula (3)(a)according to Reaction Scheme (III). To balance the charge from thiselectron transfer, a cation (e.g., H⁺) is transported across theseparator disposed between the redox active compositions comprising theredox active compounds. During discharge, when electricity is utilizedfrom the flow battery, the current direction is reversed and redoxspecies (1)(b) and (3)(a) are regenerated.

Without wishing to be bound be specific theory, it is envisaged thatcomparably electron-poor 1-ring benzoquinones may be particularlysuitable as high reduction potential redox active compounds inposilytes. While the present invention thus envisages the provision of acombination comprising a first redox active composition, which includesat least one first redox active compound characterized by any of GeneralFormulas (1), (2) or (3), it may be preferable to provide compoundscharacterized by General Formulas (1) or (2), and particularlypreferable to provide compounds characterized by General Formula (1), asfirst redox active compound(s), or mixtures thereof.

The first redox active composition may thus preferably comprise at leastone first redox active benzoquinone compound as characterized by GeneralFormula (1), optionally including at least one reduction and/oroxidation product thereof as characterized by General Formula (1)(a) or(b); or mixtures of several different benzoquinone compounds ascharacterized by General Formula (1) and optionally at least onereduction and/or oxidation product thereof.

Furthermore, without wishing to be bound by specific theory, it isenvisaged that comparably electron-rich anthraquinones may beparticularly suitable as low reduction potential redox active compoundsin negolytes. While the present invention thus envisages the provisionof a combination comprising a second redox active composition, whichincludes at least one second redox active compound characterized by anyof General Formulas (1), (2) or (3), it may be preferable to providecompounds characterized by General Formulas (2) or (3), and particularlypreferable to provide compounds characterized by General Formula (3), assecond redox active compound(s), or mixtures thereof.

The second redox active composition may thus preferably comprise atleast one second redox active anthraquinone compound characterized bygeneral formula (3), optionally including at least one reduction and/oroxidation product thereof as characterized by general formula (3)(a) or(b); or

at least one second redox active naphthoquinone compound characterizedby general formula (2), optionally including at least one reductionand/or oxidation product thereof as characterized by general formula(2)(a) or (b); or mixtures thereof.

3.3 Quinone Compounds: Substitution

Redox active quinone compounds contained in the first and second redoxactive composition (preferably used as posilyte and negolyte in theinventive redox flow batteries, respectively) of the inventivecombination may preferably be substituted (i.e., at least one of the “R”groups may be selected from a group which is different from H).

Substitution may preferably alter or confer important characteristicsincluding solubility, stability, redox kinetics, toxicity, and potentialor current market price.

Solubility may be important because the mass transport limitation athigh current density in a redox flow battery is directly proportional tothe solubility. An increased solubility may advantageously increase theworking concentration of the redox active compounds, reduce solventcosts and increase the energy density per unit volume/weight. Thecapacity of a redox flow battery depends on the effective concentrationof redox active compounds, which is the solubility multiplied by thenumber of electrons transferred in the redox reactions. Highly solubleelectrolytes therefore preferably increase the energy capacity of theredox flow battery and are therefore preferred.

The redox active compositions of the inventive combination maypreferably comprise quinone compounds according to General Formula (1),(2) or (3) in aqueous solution. Generally, unsubstituted quinonecompounds may exhibit a limited solubility in water. Water solubilitymay be enhanced by attaching polar groups such as ether, polyether,ester, sulfonyl or hydroxyl groups. Examples of such functional groupsinclude, but are not limited to, —SO₃H/SO₃, —PO₃H₂/—PO₃H⁻/—PO₃ ²⁻,—COOH/—COO⁻, —OH/—O⁻, pyridinyl, imidazoyl, —NH₂/NH₃ ⁺, —NHR/NH₂R⁺,—NR₂/NHR₂ ⁺ and —NR₃ ⁺, where R is H or optionally substituted C₁₋₆alkyl optionally comprising at least one heteroatom selected from N, Oand S, including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, —C_(n)H_(2n)NR₂,—C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H, wherein n is an integer selectedfrom 1, 2, 3, 4, 5, or 6, where R is H or optionally substituted C₁₋₆alkyl optionally comprising at least one heteroatom selected from N, Oand S, including —C_(n)H_(2n)OH, —C_(n)H_(2n), NH₂, C_(n)H_(2n)CO₂H and—C_(n)H_(2n)SO₃H. Solubility-increasing groups may advantageously beintroduced into redox active (quinone) compounds in order to increasetheir solubility. The resulting redox active composition comprising suchcompounds (the first or second redox active composition of the inventivecombination) may advantageously be used as the posilyte or negolyte inthe inventive redox flow batteries.

Stability may be important not only to prevent chemical loss for longcycle life, but also because polymerization on the electrode cancompromise the electrode's effectiveness. Stability against water andpolymerization may be enhanced by replacing C—H groups (in particularthose adjacent to C═O groups) with stable groups, e.g. selected fromoptionally substituted C₁₄ alkyl, optionally substituted C₁₋₆ alkoxy,hydroxyl, sulfonyl, amino, nitro, carboxyl, phosphoryl or phosphonyl.

Redox kinetics may be altered by adding electron-withdrawing groups (inorder to preferably increase the standard reduction potential of theresulting substituted compound) or electron-donating groups (in order topreferably lower the standard reduction potential of the resultingsubstituted compound). Electron-withdrawing groups may be selected from—SO₃H/—SO₃ ⁻, —OH/—O⁻, —COR, —COOR, —NO₂, —NR₃ ⁺, —CF₃, —CCl₃, —CN,—PO₃H₂/—PO₃H⁻/—PO₃ ²⁻, —COOH/—COO⁻, —F, —Cl, —Br, —CHO, where R is H orC₁₋₁₀alkyl. Electron-withdrawing groups may advantageously be introducedinto redox active (quinone) compounds in order to increase theirstandard reduction potential. The resulting redox active compositioncomprising such compounds (which may preferably be the first redoxactive composition of the inventive combination) may advantageously beused as the posilyte in the inventive redox flow batteries.Electron-donating groups may be selected from C₁.alkyl, including methyl(—CH₃), ethyl (—C₂H₅), or phenyl, —NH₂, —NHR, —NR₂, —NHCOR, —OR, where Ris H or C₁₋₁₀alkyl. Electron-donating groups may advantageously beintroduced into redox active (quinone) compounds in order to lower theirstandard reduction potential. The resulting redox active compositioncomprising such compounds (which may preferably be the second redoxactive composition of the inventive combination) may advantageously beused as the negolyte in the inventive redox flow batteries.

It should be appreciated that the redox active (quinone) compoundsdisclosed herein can be used for preparing either the posilyte or thenegolyte in the inventive redox flow battery. This depends on therelative standard electrode potentials of the redox active compoundspresent in the electrolytes. Moreover, each of the redox active(quinone) compounds may be used independently from the disclosedcombinations, e.g. in redox flow batteries deploying a redox active(quinone) compound as disclosed herein as a redox active compound on oneelectrode side of the redox flow battery cell, and a different redoxactive compound having a higher or lower standard reduction potential onthe other electrode side of the redox flow battery cell.

Preferred quinone compounds for preparing the redox active compositionsof the inventive combination (preferably used as posilyte and negolyteof the inventive redox flow batteries, respectively) are preferablysoluble in water, chemically stable and exhibit standard reductionpotentials as defined elsewhere herein.

More preferably, quinone compounds used in the redox active compositionsof the inventive combination are highly soluble in water, chemicallystable in strongly acidic/basic solutions, and, when used in redox flowbatteries, capable of providing high cell voltages of about 1 V,round-trip efficiencies >80%, and high discharge rates.

Accordingly, preferred quinone compounds for preparing the first andsecond redox active composition of the inventive combination (which arepreferably used as posilyte and negolyte of the inventive redox flowbattery, respectively) may comprise electron-withdrawing orelectron-donating groups for increasing or lowering the standardreduction potential (depending on whether the resulting composition isenvisaged for use as a posilyte or negolyte, respectively) andoptionally further substituents increasing their solubility in water. Inprinciple, the said redox active quinone compounds may comprise thesesubstituents in any suitable combination.

Preferred (substituted) redox active compounds are specified below.

Preferably, in redox active compounds according to General Formula (1):

R¹ may be selected from —H, —SO₃H, optionally substituted C₁₋₆ alkyl andoptionally substituted amine; R² may be selected from —H, —OH, —SO₃H,optionally substituted amine and C₁₋₆ alkoxy, preferably methoxy; R³ maybe selected from —H, —OH and C₁₋₆ alkoxy, preferably methoxy; and R⁴ maybe selected from —H, —SO₃H, optionally substituted C₁₋₆ alkyl,optionally substituted amine and halogen.

As indicated elsewhere herein, alkyl and alkoxy groups, in particularC₁₋₆ alkyl and alkoxy groups disclosed in connected with GeneralFormulas (1), (2) and (3) herein, may be linear or branched, andoptionally substituted or unsubstituted.

More preferably, in redox active compounds according to General Formula(1), R¹ and/or R⁴ may be independently selected from substituted C₁₋₆alkyl selected from —R⁵—SO₃H, —R⁵—CO₂H and R⁵—OH, wherein R⁵ is C₁ alkyloptionally comprising at least one, optionally substituted, heteroatomselected from N, O or S; or R¹, R² and/or R³ according to GeneralFormula (1) may be selected from —NH₂/NH₃ ⁺, —NHR/NH₂R⁺, —NR₂/NHR₂ ⁺ and—NR₃ ⁺, where R is H or optionally substituted C₁₋₆ alkyl, optionallycomprising at least one heteroatom selected from N, O and S, including—C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, —C_(n)H_(2n)NR₂, —C_(n)H_(2n)CO₂H and—C_(n)H_(2n)SO₃H, wherein n is an integer selected from 1, 2, 3, 4, 5,or 6, where R is H or optionally substituted C₁₋₆alkyl optionallycomprising at least one heteroatom selected from N, O and S, including—C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H.

In particular embodiments, redox active compounds according to GeneralFormula (1) may be characterized one of the following StructuralFormulas (1.1)-(1.10), or the corresponding quinone forms thereof:

Preferably, in redox active compounds according to General Formula (2),R¹ and R² may be independently selected from —H, —OH and C₁₋₆ alkoxy,preferably methoxy; and R³—R⁶ may be independently selected from —H and—SO₃H.

According to the alternative annotation provided herein, in redox activecompounds according to General Formula (2), R⁵ and R⁶ may beindependently selected from —H, —OH and C₁₋₆ alkoxy, preferably methoxy;and R⁷—R¹⁰ may be independently selected from —H and —SO₃H.

Preferably, in redox active compounds according to General Formula (3),R¹, R² and R may be independently selected from —H, —OH and C₁₋₆ alkoxy,preferably methoxy; and R³ and R⁵-R⁸ may be independently selected from—H and —SO₃H.

More preferably, in redox active compounds according to General Formula(3), R¹ may be —SO₃H; R² may be —SO₃H and R¹, R³ and R⁴ may preferablybe —OH; R⁶ may be —SO₃H, R¹ and R⁴ or R¹, R² and R⁴ may preferably be—OH; R² and R⁶ may be —SO₃H, R¹ and R⁴ or R¹, R³ and R⁴ may preferablybe —OH; R³ and R⁶ may be —SO₃H; R¹, R² and R⁴ may preferably be —OH; R²and R⁷ may be —SO₃H; or R¹ and R⁴ are —SO₃H; wherein each of the othersof R¹-R⁸ may be C₁₋₆ alkoxy or —H, preferably —H.

According to the alternative annotation used herein, preferably, inredox active compounds according to General Formula (3), R¹¹, R¹² andR¹⁴ may be independently selected from —H, —OH and C₁₋₆ alkoxy,preferably methoxy; and R¹³ and R¹⁵-R¹⁸ may be independently selectedfrom —H and —SO₃H.

More preferably, in redox active compounds according to General Formula(3), R¹¹ may be —SO₃H; R¹² may be —SO₃H and R¹¹, R¹³ and R¹⁴ maypreferably be —OH; R¹⁶ may be —SO₃H, R¹¹ and R¹⁴ or R¹¹, R¹² and R¹⁴ maypreferably be —OH; R¹² and R¹⁶ may be —SO₃H, R¹¹ and R¹⁴ or R¹¹, R¹³ andR¹⁴ may preferably be —OH; R¹³ and R¹⁶ may be —SO₃H; R¹¹, R¹² and R¹⁴may preferably be —OH; R¹² and R¹⁷ may be —SO₃H; or R¹¹ and R¹⁴ are—SO₃H; wherein each of the others of R¹¹-R¹⁸ may be C₁₋₆ alkoxy or —H,preferably —H.

In particular embodiments, redox active compounds according to GeneralFormula (3) may be characterized by Structural Formula (6.1), or thecorresponding hydroquinone form thereof:

In preferred embodiments, the inventive combination may thus comprise afirst redox active compound selected from at least one benzohydroquinonecharacterized by formula (1.1)-(1.6) or (1.9), or mixtures thereof, andoptionally oxidation products thereof; and a second redox activecompound selected from preferably at least one anthraquinonecharacterized by formula (6.1) or mixtures thereof, and optionallyreduction products thereof; or at least one benzohydroquinonecharacterized by formula (1.7) or (1.8) or (1.10) or mixtures thereof,or optionally reduction products thereof.

Further preferred redox active compounds useful for preparing the firstand/or second redox active composition of the present invention(preferably used as posilyte or negolyte, respectively, in the inventiveredox flow battery) include 1,4-benzoquinone-2,5-disulfonic acid,1,4-benzoquinone-2,6-disulfonic acid, 1,4-benzoquinone-2-sulfonic acid,1,4-naphthoquinone-2,6-disulfonic acid,1,4-naphthoquinone-2,7-disulfonic acid,1,4-naphthoquinone-5,7-disulfonic acid, 1,4-naphthoquinone-5-sulfonicacid, 1,4-naphthoquinone-2-sulfonic acid,9,10-anthraquinone-2,6-disulfonic acid,9,10-anthraquinone-2,7-disulfonic acid,9,10-anthraquinone-1,5-disulfonic acid, 9,10-anthraquinone-1-sulfonicacid and 9,10-anthraquinone-2-sulfonic acid, or reduction productsthereof.

Further preferred redox active compounds according to General Formula(1), which are useful for preparing the first and/or second redox activecomposition of the present invention (preferably used as posilyte ornegolyte, respectively, in the inventive redox flow battery) are listedin Table 1 below. In further preferred compounds according to table 1,R¹ and R⁴ may be —SO₃H.

TABLE 1 Preferred structures for benzoquinone and benzohydroquinonederivatives:

C₁₋₆-alkoxy ID SO₃H substituents OH substituents substituted Alkylsubstituents position amount position amount position amount positionamount 1 R¹ Mono- — None — None — None 2 R¹-R⁴ Di- — None — None — None3 R¹-R⁴ Tri- — None — None — None 4 R¹ Mono- — None R²-R⁴ Mono- — None 5R¹ Mono- — None — None R²-R⁴ Mono- 6 R¹ Mono- — None R²-R⁴ Mono- R²-R⁴Mono- 7 R¹ Mono- — None R²-R³ Di- — None 8 R¹ Mono- — None — None R²-R⁴Di- 9 R¹ Mono- — None R²-R³ Di- R²-R⁴ Mono- 10 R¹ Mono- — None R²-R⁴Mono- R²-R⁴ Di- 11 R¹-R⁴ Di- — None R²-R⁴ Mono- — None 12 R¹-R⁴ Di- —None — None R²-R⁴ Mono- 13 R¹-R⁴ Di- — None R²-R³ Di- — None 14 R¹-R⁴Di- — None — None R²-R⁴ Di- 15 R¹-R⁴ Tri- — None R²-R⁴ Mono- — None 16R¹-R⁴ Tri- — None — None R²-R⁴ Mono-

Particularly preferred benzoquinone and benzohydroquinone derivativesare molecules with ID No. 1-3 and 11-16.

Further preferred redox active compounds according to General Formula(2), which are useful for preparing the first and/or second redox activecomposition of the present invention (preferably used as posilyte ornegolyte, respectively, in the inventive redox flow battery) are listedin Table 2 below. In preferred compounds according to table 2, R³ may be—SO₃H. In further preferred compounds according to table 2, R⁴ may be—SO₃H. In further preferred compounds according to table 2, R⁵ may be—SO₃H. In further preferred compounds according to table 2, R⁶ may be—SO₃H.

TABLE 2 Preferred structures for naphthoquinone and naphthohydroquinonederivatives:

C₁₋₆-alkoxy ID SO₃H substituents OH substituents substituted Alkylsubstituents position amount position amount position amount positionamount 17 R¹, R³, R⁴ Mono- — None — None — None 18 R¹-R⁶ Di- — None —None — None 19 R¹-R⁶ Tri- — None — None — None 20 R¹-R⁶ Tetra- — None —None — None 21 R¹-R⁶ Penta- — None — None — None 22 R¹, R³, R⁴ Mono- —None R¹-R⁶ Mono- — None 23 R¹, R³, R⁴ Mono- — None — None R¹-R⁶ Mono- 24R¹, R³, R⁴ Mono- — None R¹-R⁶ Mono- R¹-R⁶ Mono- 25 R¹, R³, R⁴ Mono- R³,R⁶ Di- — None — None 26 R¹, R³, R⁴ Mono- — None R¹-R⁶ Di- — None 27 R¹,R³, R⁴ Mono- — None — None R1-R⁶ Di- 28 R¹, R³, R⁴ Mono- R³, R⁶ Di-R¹⁻²-R⁴⁻⁵ Mono- — None 35 R¹, R³, R⁴ Mono- R³, R⁶ Di- — None R¹⁻²-R⁴⁻⁵Mono- 36 R¹, R³, R⁴ Mono- R³, R⁶ Di- R¹⁻²-R⁴⁻⁵ Mono- R¹⁻²-R⁴⁻⁵ Mono- 37R¹, R³, R⁴ Mono- R³, R⁶ Di- R¹⁻²-R⁴⁻⁵ Di- — None 38 R¹, R³, R⁴ Mono- R³,R⁶ Di- — None R¹⁻²-R⁴⁻⁵ Di- 39 R¹, R³, R⁴ Mono- R³, R⁶ Di- R¹⁻²-R⁴⁻⁵ Di-R¹⁻²-R⁴⁻⁵ Mono- 40 R¹, R³, R⁴ Mono- R³, R⁶ Di- R¹⁻²-R⁴⁻⁵ Mono- R¹⁻²-R⁴⁻⁵Di- 35 R¹, R³, R⁴ Mono- — None R1-R⁶ Di- R¹-R⁶ Mono- 36 R¹, R³, R⁴ Mono-— None R¹-R⁶ Mono- R1-R⁶ Di- 37 R¹, R³, R⁴ Mono- — None R1-R⁶ Di- R1-R⁶Di- 38 R¹, R³, R⁴ Mono- — None R1-R⁶ Tri- — None 39 R¹, R³, R⁴ Mono- —None — None R1-R⁶ Tri- 40 R¹, R³, R⁴ Mono- — None R1-R⁶ Tri- R¹-R⁶ Mono-41 R¹, R³, R⁴ Mono- — None R¹-R⁶ Mono- R1-R⁶ Tri- 42 R¹, R³, R⁴ Mono- —None R1-R⁶ Tri- R¹-R⁶ Di- 43 R¹, R³, R⁴ Mono- — None R¹-R⁶ Di- R1-R⁶Tri- 44 R¹, R³, R⁴ Mono- — None R1-R⁶ Tetra- — None 45 R¹, R³, R⁴ Mono-— None — None R1-R⁶ Tera- 46 R¹, R³, R⁴ Mono- — None R1-R⁶ Tetra- R¹-R⁶Mono- 47 R¹, R³, R⁴ Mono- — None R¹-R⁶ Mono- R1-R⁶ Tetra- 48 R¹- R⁶ Di-— None R¹-R⁶ Mono- — None 49 R¹- R⁶ Di- — None — None R¹-R⁶ Mono- 50 R¹-R⁶ Di- — None R¹-R⁶ Mono- R¹-R⁶ Mono- 51 R¹- R⁶ Di- R³, R⁶ Di- — None —None 52 R¹- R⁶ Di- — None R1-R⁶ Di- — None 53 R¹- R⁶ Di- — None — NoneR1-R⁶ Di- 54 R¹- R⁶ Di- R³, R⁶ Di- R¹⁻²-R⁴⁻⁵ Mono- — None 55 R¹- R⁶ Di-R³, R⁶ Di- — None R¹⁻²-R⁴⁻⁵ Mono- 56 R¹- R⁶ Di- R³, R⁶ Di- R¹⁻²-R⁴⁻⁵Mono- R¹⁻²-R⁴⁻⁵ Mono- 57 R¹- R⁶ Di- R³, R⁶ Di- R¹⁻²-R⁴⁻⁵ Di- — None 58R¹- R⁶ Di- R³, R⁶ Di- — None R¹⁻²-R⁴⁻⁵ Di- 59 R¹- R⁶ Di- — None R1-R⁶Di- R¹-R⁶ Mono- 60 R¹- R⁶ Di- — None R¹-R⁶ Mono- R1-R⁶ Di- 61 R¹- R⁶ Di-— None R1-R⁶ Di- R1-R⁶ Di- 62 R¹- R⁶ Di- — None R1-R⁶ Tri- — None 63 R¹-R⁶ Di- — None — None R1-R⁶ Tri- 64 R¹- R⁶ Di- — None R1-R⁶ Tri- R¹-R⁶Mono- 65 R¹- R⁶ Di- — None R¹-R⁶ Mono- R1-R⁶ Tri- 66 R¹- R⁶ Tri- — NoneR¹-R⁶ Mono- — None 67 R¹- R⁶ Tri- — None — None R¹-R⁶ Mono- 68 R¹- R⁶Tri- — None R¹-R⁶ Mono- R¹-R⁶ Mono- 69 R¹- R⁶ Tri- R³, R⁶ Di- — None —None 70 R¹- R⁶ Tri- — None R1-R⁶ Di- — None 71 R¹- R⁶ Tri- — None — NoneR1-R⁶ Di- 72 R¹- R⁶ Tri- R³, R⁶ Di- R¹⁻²-R⁴⁻⁵ Mono- — None 73 R¹- R⁶Tri- R³, R⁶ Di- — None R¹⁻²-R⁴⁻⁵ Mono- 74 R¹- R⁶ Tri- — None R1-R⁶ Di-R¹-R⁶ Mono- 75 R¹- R⁶ Tri- — None R¹-R⁶ Mono- R1-R⁶ Di- 76 R¹- R⁶ Tri- —None R1-R⁶ Tri- — None 77 R¹- R⁶ Tri- — None — None R1-R⁶ Tri- 78 R¹- R⁶Tetra- — None R¹-R⁶ Mono- — None 79 R¹- R⁶ Tetra- — None — None R¹-R⁶Mono- 80 R¹- R⁶ Tetra- — None R¹-R⁶ Mono- R¹-R⁶ Mono- 81 R¹- R⁶ Tetra-R³, R⁶ Di- — None — None 82 R¹- R⁶ Tetra- — None R1-R⁶ Di- — None 83 R¹-R⁶ Tetra- — None — None R1-R⁶ Di- 84 R¹- R⁶ Penta- — None R¹-R⁶ Mono- —None 85 R¹- R⁶ Penta- — None — None R¹-R⁶ Mono-

Particularly preferred naphthoquinone and naphthohydroquinonederivatives are molecules with ID No. 17-19, 22-23, 48-49, 52-53, 59-61,66-68, 70-71, 74-75.

Further preferred redox active compounds according to General Formula(3), which are useful for preparing the first and/or second redox activecomposition of the present invention (preferably used as posilyte ornegolyte, respectively, in the inventive redox flow battery) are listedin Table 3 below. In preferred compounds according to table 3, R³ may beSO₃H. In further preferred compounds according to table 3, R⁵ may beSO₃H. In further preferred compounds according to table 3, R⁶ may beSO₃H. In further preferred compounds according to table 3, R⁷ may beSO₃H. In further preferred compounds according to table 3, R⁸ may beSO₃H.

TABLE 3 Preferred structures for anthraquinone and anthrahydroquinoneDerivatives:

C₁₋₆-alkoxy ID SO₃H substituents OH substituents substituted Alkylsubstituents position amount position amount position amount positionamount 86 R¹⁻² Mono- — None — None — None 87 R¹-R⁸ Di- — None — None —None 88 R¹-R⁸ Tri- — None — None — None 89 R¹-R⁸ Tetra- — None — None —None 90 R¹-R⁸ Penta- — None — None — None 91 R¹⁻² Mono- R¹-R⁸ Mono- —None — None 92 R¹⁻² Mono- — None R¹-R⁸ Mono- — None 93 R¹⁻² Mono- — None— None R¹-R⁸ Mono- 94 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Mono- — None 95 R¹⁻²Mono- R¹-R⁸ Mono- — None R¹-R⁸ Mono- 96 R¹⁻² Mono- — None R¹-R⁸ Mono-R¹-R⁸ Mono- 97 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Mono- R¹-R⁸ Mono- 98 R¹⁻²Mono- R¹⁻⁸ Di- — None — None 99 R¹⁻² Mono- — None R¹-R⁸ Di- — None 100R¹⁻² Mono- — None — None R¹-R⁸ Di- 101 R¹⁻² Mono- R¹-R⁸ Di- R¹-R⁸ Mono-— None 102 R¹⁻² Mono- R¹-R⁸ Di- — None R¹-R⁸ Mono- 103 R¹⁻² Mono- R³, R⁶Di- R¹-R⁸ Mono- R¹-R⁸ Mono- 104 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Di- — None105 R¹⁻² Mono- — None R¹-R⁸ Di- R¹-R⁸ Mono- 106 R¹⁻² Mono- R¹-R⁸ Mono-R¹-R⁸ Di- R¹-R⁸ Mono- 107 R¹⁻² Mono- R¹-R⁸ Mono- — None R¹-R⁸ Di- 108R¹⁻² Mono- — None R¹-R⁸ Mono- R¹-R⁸ Di- 109 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸Mono- R¹-R⁸ Di- 110 R¹⁻² Mono- R¹-R⁸ Di- R¹-R⁸ Di- — None 111 R¹⁻² Mono-R¹-R⁸ Di- R¹-R⁸ Di- R¹-R⁸ Mono- 112 R¹⁻² Mono- R¹-R⁸ Di- — None R¹-R⁸Di- 113 R¹⁻² Mono- R¹-R⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Di- 114 R¹⁻² Mono- — NoneR¹-R⁸ Di- R¹-R⁸ Di- 115 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Di- R¹-R⁸ Di- 116R¹⁻² Mono- R¹-R⁸ Di- R¹-R⁸ Di- R¹-R⁸ Di- 117 R¹⁻² Mono- R¹⁻⁸ Tri- — None— None 118 R¹⁻² Mono- — None R¹-R⁸ Tri — None 119 R¹⁻² Mono- — None —None R¹-R⁸ Tri 120 R¹⁻² Mono- R¹-R⁸ Tri- R¹-R⁸ Mono- — None 121 R¹⁻²Mono- R¹-R⁸ Tri- — None R¹-R⁸ Mono- 122 R¹⁻² Mono- R³, R⁶ Tri- R¹-R⁸Mono- R¹-R⁸ Mono- 123 R¹⁻² Mono- R¹-R⁸ Tri- R¹-R⁸ Di- — None 124 R¹⁻²Mono- R¹-R⁸ Tri- — None R¹-R⁸ Di- 125 R¹⁻² Mono- R¹-R⁸ Tri- R¹-R⁸ Di-R¹-R⁸ Mono- 126 R¹⁻² Mono- R¹-R⁸ Tri- R¹-R⁸ Mono- R¹-R⁸ Di- 127 R¹⁻²Mono- R³, R⁶ Tri- R¹-R⁸ Di- R¹-R⁸ Di- 128 R¹⁻² Mono- R¹-R⁸ Tri- R¹-R⁸Tri- — None 129 R¹⁻² Mono- R¹-R⁸ Tri- — None R¹-R⁸ Tri- 130 R¹⁻² Mono-R¹-R⁸ Tri- R¹-R⁸ Tri- R¹-R⁸ Mono- 131 R¹⁻² Mono- R³, R⁶ Tri- R¹-R⁸ Mono-R¹-R⁸ Tri- 132 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Tri- — None 133 R¹⁻² Mono- —None R¹-R⁸ Tri- R¹-R⁸ Mono- 134 R¹⁻² Mono- R¹-R⁸ Mono- R³, R⁶ Tri- R¹-R⁸Mono- 135 R¹⁻² Mono- R¹-R⁸ Di- R¹-R⁸ Tri- — None 136 R¹⁻² Mono- — NoneR¹-R⁸ Tri- R¹-R⁸ Di- 137 R¹⁻² Mono- R¹-R⁸ Di- R¹-R⁸ Tri- R¹-R⁸ Mono- 138R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Tri- R¹-R⁸ Di- 139 R¹⁻² Mono- R¹-R⁸ Di- R³,R⁶ Tri- R¹-R⁸ Di- 140 R¹⁻² Mono- — None R¹-R⁸ Tri- R¹-R⁸ Tri- 141 R¹⁻²Mono- R¹-R⁸ Mono- R¹-R⁸ Tri- R¹-R⁸ Tri- 142 R¹⁻² Mono- R¹-R⁸ Mono- —None R¹-R⁸ Tri- 143 R¹⁻² Mono- — None R¹-R⁸ Mono- R¹-R⁸ Tri- 144 R¹⁻²Mono- R¹-R⁸ Mono- R¹-R⁸ Mono- R³, R⁶ Tri- 145 R¹⁻² Mono- R¹-R⁸ Di- —None R¹-R⁸ Tri- 146 R¹⁻² Mono- — None R¹-R⁸ Di- R¹-R⁸ Tri- 147 R¹⁻²Mono- R¹-R⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Tri- 148 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸Di- R¹-R⁸ Tri- 149 R¹⁻² Mono- R¹-R⁸ Di- R¹-R⁸ Di- R³, R⁶ Tri- 150 R¹⁻²Mono- R¹⁻⁸ Quart- — None — None 151 R¹⁻² Mono- — None R¹-R⁸ Quart- —None 152 R¹⁻² Mono- — None — None R1-R⁸ Quart- 153 R¹⁻² Mono- R¹⁻⁸Quart- R¹-R⁸ Mono- — None 154 R¹⁻² Mono- R¹⁻⁸ Quart- — None R¹-R⁸ Mono-155 R¹⁻² Mono- R³, R⁶ Quart- R¹-R⁸ Mono- R¹-R⁸ Mono- 156 R¹⁻² Mono- R¹⁻⁸Quart- R¹-R⁸ Di- — None 157 R¹⁻² Mono- R¹⁻⁸ Quart- — None R¹-R⁸ Di- 158R¹⁻² Mono- R¹⁻⁸ Quart- R¹-R⁸ Di- R¹-R⁸ Mono- 159 R¹⁻² Mono- R¹⁻⁸ Quart-R¹-R⁸ Mono- R¹-R⁸ Di- 160 R¹⁻² Mono- R¹⁻⁸ Quart- R¹-R⁸ Tri- — None 161R¹⁻² Mono- R¹⁻⁸ Quart- — None R¹-R⁸ Tri- 162 R¹⁻² Mono- R¹⁻⁸ Mono- R¹-R⁸Quart- — None 163 R¹⁻² Mono- — None R¹-R⁸ Quart- R¹-R⁸ Mono- 164 R¹⁻²Mono- R¹⁻⁸ Mono- R³, R⁶ Quart- R¹-R⁸ Mono- 165 R¹⁻² Mono- R¹⁻⁸ Di- R¹-R⁸Quart- — None 166 R¹⁻² Mono- — None R¹-R⁸ Quart- R¹-R⁸ Di- 167 R¹⁻²Mono- R¹-R⁸ Di- R¹-R⁸ Quart- R¹-R⁸ Mono- 168 R¹⁻² Mono- R¹-R⁸ Mono-R¹-R⁸ Quart- R¹-R⁸ Di- 169 R¹⁻² Mono- — None R¹-R⁸ Quart- R¹-R⁸ Tri- 170R¹⁻² Mono- R¹-R⁸ Tri R¹-R⁸ Quart- — None 171 R¹⁻² Mono- R¹-R⁸ Mono- —None R¹-R⁸ Quart- 172 R¹⁻² Mono- — None R¹-R⁸ Mono- R¹-R⁸ Quart- 173R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Mono- R³, R⁶ Quart- 174 R¹⁻² Mono- R¹-R⁸Di- - None R¹-R⁸ Quart- 175 R¹⁻² Mono- — None R¹-R⁸ Di- R¹-R⁸ Quart- 176R¹⁻² Mono- R¹-R⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Quart- 177 R¹⁻² Mono- R¹-R⁸ Mono-R¹-R⁸ Di- R¹-R⁸ Quart- 178 R¹⁻² Mono- R¹-R⁸ Tri- — None R¹-R⁸ Quart- 179R¹⁻² Mono- — None R¹-R⁸ Tri- R¹-R⁸ Quart- 180 R¹⁻² Mono- R¹⁻⁸ Pent- —None — None 181 R¹⁻² Mono- — None R¹-R⁸ Pent- — None 182 R¹⁻² Mono- —None — None R1-R⁸ Pent- 183 R¹⁻² Mono- R¹-R⁸ Pent- R¹-R⁸ Mono- — None184 R¹⁻² Mono- R¹-R⁸ Pent- — None R¹-R⁸ Mono- 185 R¹⁻² Mono- R³, R⁶Pent- R¹-R⁸ Mono- R¹-R⁸ Mono- 186 R¹⁻² Mono- R¹-R⁸ Pent- R¹-R⁸ Di- —None 187 R¹⁻² Mono- R¹-R⁸ Pent- — None R¹-R⁸ Di- 188 R¹⁻² Mono- R¹-R⁸Mono- R¹-R⁸ Pent- — None 189 R¹⁻² Mono- — None R¹-R⁸ Pent- R¹-R⁸ Mono-190 R¹⁻² Mono- R¹-R⁸ Mono- R³, R⁶ Pent- R¹-R⁸ Mono- 191 R¹⁻² Mono- R¹-R⁸Di- R¹-R⁸ Pent- — None 192 R¹⁻² Mono- — None R¹-R⁸ Pent- R¹-R⁸ Di- 193R¹⁻² Mono- R¹-R⁸ Mono- — None R¹-R⁸ Pent- 194 R¹⁻² Mono- — None R¹-R⁸Mono- R¹-R⁸ Pent- 195 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Mono- R³-R⁶ Pent- 196R¹⁻² Mono- R¹-R⁸ Di- — None R¹-R⁸ Pent- 197 R¹⁻² Mono- — None R¹-R⁸ Di-R¹-R⁸ Pent- 198 R¹⁻² Mono- R¹⁻⁸ Hexa- — None — None 199 R¹⁻² Mono- —None R¹-R⁸ Hexa- — None 200 R¹⁻² Mono- — None — None R1-R⁸ Hexa- 201R¹⁻² Mono- R¹-R⁸ Hexa- R¹-R⁸ Mono- — None 202 R¹⁻² Mono- R¹-R⁸ Hexa- —None R¹-R⁸ Mono- 203 R¹⁻² Mono- R¹-R⁸ Mono- R¹-R⁸ Hexa- — None 204 R¹⁻²Mono- — None R¹-R⁸ Hexa- R¹-R⁸ Mono- 205 R¹⁻² Mono- R¹-R⁸ Mono- — NoneR¹-R⁸ Hexa- 206 R¹⁻² Mono- — None R¹-R⁸ Mono- R¹-R⁸ Hexa- 207 R¹⁻² Mono-R¹⁻⁸ Hepta- — None — None 208 R¹⁻² Mono- — None R¹-R⁸ Hepta- — None 209R¹⁻² Mono- — None — None R1-R⁸ Hepta- 210 R¹⁻⁸ Di- R¹-R⁸ Mono- — None —None 211 R¹⁻⁸ Di- — None R¹-R⁸ Mono- — None 212 R¹⁻⁸ Di- — None — NoneR¹-R⁸ Mono- 213 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Mono- - None 214 R¹⁻⁸ Di-R¹-R⁸ Mono- — None R¹-R⁸ Mono- 215 R¹⁻⁸ Di- — None R¹-R⁸ Mono- R¹-R⁸Mono- 216 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Mono- R¹-R⁸ Mono- 217 R¹⁻⁸ Di- R¹⁻⁸Di- — None — None 218 R¹⁻⁸ Di- — None R¹-R⁸ Di- — None 219 R¹⁻⁸ Di- —None — None R1-R⁸ Di- 220 R¹⁻⁸ Di- R¹-R⁸ Di- R¹-R⁸ Mono- — None 221 R¹⁻⁸Di- R¹-R⁸ Di- — None R¹-R⁸ Mono- 223 R¹⁻⁸ Di- R³, R⁶ Di- R¹-R⁸ Mono-R¹-R⁸ Mono- 224 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Di- — None- 225 R¹⁻⁸ Di- —None R¹-R⁸ Di- R¹-R⁸ Mono- 226 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Di- R¹-R⁸Mono- 227 R¹⁻⁸ Di- R¹-R⁸ Mono- — None R¹-R⁸ Di- 228 R¹⁻⁸ Di- — NoneR¹-R⁸ Mono- R¹-R⁸ Di- 229 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Mono- R¹-R⁸ Di- 230R¹⁻⁸ Di- R¹-R⁸ Di- R¹-R⁸ Di- — None 231 R¹⁻⁸ Di- R¹-R⁸ Di- R¹-R⁸ Di-R¹-R⁸ Mono- 232 R¹⁻⁸ Di- R¹-R⁸ Di- — None R¹-R⁸ Di- 233 R¹⁻⁸ Di- R¹-R⁸Di- R¹-R⁸ Mono- R¹-R⁸ Di- 234 R¹⁻⁸ Di- — None R¹-R⁸ Di- R¹-R⁸ Di- 235R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Di- R¹-R⁸ Di- 236 R¹⁻⁸ Di- R¹-R⁸ Di- R¹-R⁸Di- R¹-R⁸ Di- 237 R¹⁻⁸ Di- R¹⁻⁸ Tri- — None — None 238 R¹⁻⁸ Di- — NoneR¹-R⁸ Tri — None 239 R¹⁻⁸ Di- — None — None R1-R⁸ Tri 240 R¹⁻⁸ Di- R¹-R⁸Tri- R¹-R⁸ Mono- — None 241 R¹⁻⁸ Di- R¹-R⁸ Tri- — None R¹-R⁸ Mono- 242R¹⁻⁸ Di- R³, R⁶ Tri- R¹-R⁸ Mono- R¹-R⁸ Mono- 243 R¹⁻⁸ Di- R¹-R⁸ Tri-R¹-R⁸ Di- — None 244 R¹⁻⁸ Di- R¹-R⁸ Tri- — None R¹-R⁸ Di- 245 R¹⁻⁸ Di-R¹-R⁸ Tri- R¹-R⁸ Di- R¹-R⁸ Mono- 246 R¹⁻⁸ Di- R¹-R⁸ Tri- R¹-R⁸ Mono-R¹-R⁸ Di- 247 R¹⁻⁸ Di- R¹-R⁸ Tri- R¹-R⁸ Tri- — None 248 R¹⁻⁸ Di- R¹-R⁸Tri- — None R¹-R⁸ Tri- 248 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Tri- — None 249R¹⁻⁸ Di- — None R¹-R⁸ Tri- R¹-R⁸ Mono- 250 R¹⁻⁸ Di- R¹⁻⁸ Mono- R³, R⁶Tri- R¹-R⁸ Mono- 251 R¹⁻⁸ Di- R¹⁻⁸ Di- R¹-R⁸ Tri- — None 252 R¹⁻⁸ Di- —None R¹-R⁸ Tri- R¹-R⁸ Di- 253 R¹⁻⁸ Di- R¹-R⁸ Di- R¹-R⁸ Tri- R¹-R⁸ Mono-254 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Tri- R¹-R⁸ Di- 255 R¹⁻⁸ Di- — None R¹-R⁸Tri- R¹-R⁸ Tri- 256 R¹⁻⁸ Di- R¹-R⁸ Mono- — None R¹-R⁸ Tri- 257 R¹⁻⁸ Di-— None R¹-R⁸ Mono- R¹-R⁸ Tri- 258 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Mono- R³,R⁶ Tri- 259 R¹⁻⁸ Di- R¹-R⁸ Di- — None R¹-R⁸ Tri- 260 R¹⁻⁸ Di- — NoneR¹-R⁸ Di- R¹-R⁸ Tri- 261 R¹⁻⁸ Di- R¹-R⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Tri- 262R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Di- R¹-R⁸ Tri- 263 R¹⁻⁸ Di- R¹⁻⁸ Quart- —None — None 264 R¹⁻⁸ Di- — None R¹-R⁸ Quart- — None 265 R¹⁻⁸ Di- — None— None R1-R⁸ Quart- 266 R¹⁻⁸ Di- R¹-R⁸ Quart- R¹-R⁸ Mono- — None 267R¹⁻⁸ Di- R¹-R⁸ Quart- — None R¹-R⁸ Mono- 268 R¹⁻⁸ Di- R³, R⁶ Quart-R¹-R⁸ Mono- R¹-R⁸ Mono- 269 R¹⁻⁸ Di- R¹-R⁸ Quart- R¹-R⁸ Di- — None 270R¹⁻⁸ Di- R¹-R⁸ Quart- — None R¹-R⁸ Di- 271 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸Quart- — None 272 R¹⁻⁸ Di- — None R¹-R⁸ Quart- R¹-R⁸ Mono- 273 R¹⁻⁸ Di-R¹-R⁸ Mono- R³, R⁶ Quart- R¹-R⁸ Mono- 274 R¹⁻⁸ Di- R¹-R⁸ Di- R¹-R⁸Quart- — None 275 R¹⁻⁸ Di- — None R¹-R⁸ Quart- R¹-R⁸ Di- 276 R¹⁻⁸ Di-R¹-R⁸ Mono- — None R¹-R⁸ Quart- 277 R¹⁻⁸ Di- — None R¹-R⁸ Mono- R¹-R⁸Quart- 278 R¹⁻⁸ Di- R¹-R⁸ Mono- R¹-R⁸ Mono- R³, R⁶ Quart- 279 R¹⁻⁸ Di-R¹-R⁸ Di- — None R¹-R⁸ Quart- 280 R¹⁻⁸ Di- — None R¹-R⁸ Di- R¹-R⁸ Quart-281 R¹⁻⁸ Di- R¹⁻⁸ Pent- — None — None 282 R¹⁻⁸ Di- — None R¹-R⁸ Pent- —None 283 R¹⁻⁸ Di- — None — None R1-R⁸ Pent- 284 R¹⁻⁸ Di- R¹-R⁸ Pent-R¹-R⁸ Mono- — None 285 R¹⁻⁸ Di- R¹-R⁸ Pent- — None R¹-R⁸ Mono- 286 R¹⁻⁸Di- R¹-R⁸ Mono- R¹-R⁸ Pent- — None 287 R¹⁻⁸ Di- — None R¹-R⁸ Pent- R¹-R⁸Mono- 288 R¹⁻⁸ Di- R¹-R⁸ Mono- — None R¹-R⁸ Pent- 289 R¹⁻⁸ Di- — NoneR¹-R⁸ Mono- R¹-R⁸ Pent- 290 R¹⁻⁸ Di- R¹⁻⁸ Hexa- — None — None 291 R¹⁻⁸Di- — None R¹-R⁸ Hexa- — None 292 R¹⁻⁸ Di- — None — None R1-R⁸ Hexa- 293R¹⁻⁸ Tri- R¹-R⁸ Mono- — None — None 294 R¹⁻⁸ Tri- — None R¹-R⁸ Mono- —None 295 R¹⁻⁸ Tri- — None — None R¹-R⁸ Mono- 296 R¹⁻⁸ Tri- R¹-R⁸ Mono-R¹-R⁸ Mono- — None 297 R¹⁻⁸ Tri- R¹-R⁸ Mono- — None R¹-R⁸ Mono- 298 R¹⁻⁸Tri- — None R¹-R⁸ Mono- R¹-R⁸ Mono- 299 R¹⁻⁸ Tri- R¹-R⁸ Mono- R¹-R⁸Mono- R¹-R⁸ Mono- 300 R¹⁻⁸ Tri- R¹⁻⁸ Di- — None — None 301 R¹⁻⁸ Tri- —None R¹-R⁸ Di- — None 302 R¹⁻⁸ Tri- — None — None R1-R⁸ Di- 303 R¹⁻⁸Tri- R¹-R⁸ Di- R¹-R⁸ Mono- — None 304 R¹⁻⁸ Tri- R¹-R⁸ Di- — None R¹-R⁸Mono- 305 R¹⁻⁸ Tri- R³, R⁶ Di- R¹-R⁸ Mono- R¹-R⁸ Mono- 306 R¹⁻⁸ Tri-R¹-R⁸ Mono- R¹-R⁸ Di- — None 307 R¹⁻⁸ Tri- — None R¹-R⁸ Di- R¹-R⁸ Mono-308 R¹⁻⁸ Tri- R¹-R⁸ Mono- R¹-R⁸ Di- R¹-R⁸ Mono- 309 R¹⁻⁸ Tri- R¹-R⁸Mono- — None R¹-R⁸ Di- 310 R¹⁻⁸ Tri- — None R¹-R⁸ Mono- R¹-R⁸ Di- 311R¹⁻⁸ Tri- R¹-R⁸ Mono- R¹-R⁸ Mono- R¹-R⁸ Di- 312 R¹⁻⁸ Tri- R¹-R⁸ Di-R¹-R⁸ Di- — None 313 R¹⁻⁸ Tri- R¹-R⁸ Di- R¹-R⁸ Di- R¹-R⁸ Mono- 314 R¹⁻⁸Tri- R¹-R⁸ Di- — None R¹-R⁸ Di- 315 R¹⁻⁸ Tri- R¹-R⁸ Di- R¹-R⁸ Mono-R¹-R⁸ Di- 316 R¹⁻⁸ Tri- — None R¹-R⁸ Di- R¹-R⁸ Di- 317 R¹⁻⁸ Tri- R¹-R⁸Mono- R¹-R⁸ Di- R¹-R⁸ Di- 318 R¹⁻⁸ Tri- R¹⁻⁸ Tri- — None — None 319 R¹⁻⁸Tri- — None R¹-R⁸ Tri — None 320 R¹⁻⁸ Tri- — None — None R¹-R⁸ Tri 321R¹⁻⁸ Tri- R¹-R⁸ Tri- R¹-R⁸ Mono- — None 323 R¹⁻⁸ Tri- R¹-R⁸ Tri- — NoneR¹-R⁸ Mono- 324 R¹⁻⁸ Tri- R³, R⁶ Tri- R¹-R⁸ Mono- R¹-R⁸ Mono- 325 R¹⁻⁸Tri- R¹-R⁸ Tri- R¹-R⁸ Di- — None 326 R¹⁻⁸ Tri- R¹-R⁸ Tri- — None R¹-R⁸Di- 327 R¹⁻⁸ Tri- R¹-R⁸ Mono- R¹-R⁸ Tri- — None 328 R¹⁻⁸ Tri- — NoneR¹-R⁸ Tri- R¹-R⁸ Mono- 329 R¹⁻⁸ Tri- R¹-R⁸ Mono- R³, R⁶ Tri- R¹-R⁸ Mono-330 R¹⁻⁸ Tri- R¹-R⁸ Di- R¹-R⁸ Tri- — None 331 R¹⁻⁸ Tri- — None R¹-R⁸Tri- R¹-R⁸ Di- 332 R¹⁻⁸ Tri- R¹-R⁸ Mono- — None R¹-R⁸ Tri- 333 R¹⁻⁸ Tri-— None R¹-R⁸ Mono- R¹-R⁸ Tri- 334 R¹⁻⁸ Tri- R¹-R⁸ Mono- R¹-R⁸ Mono- R³,R⁶ Tri- 335 R¹⁻⁸ Tri- R¹-R⁸ Di- — None R¹-R⁸ Tri- 336 R¹⁻⁸ Tri- — NoneR¹-R⁸ Di- R¹-R⁸ Tri- 337 R¹⁻⁸ Tri- R¹⁻⁸ Quart- — None — None 338 R¹⁻⁸Tri- — None R¹-R⁸ Quart- — None 339 R¹⁻⁸ Tri- — None — None R1-R⁸ Quart-340 R¹⁻⁸ Tri- R¹-R⁸ Quart- R¹-R⁸ Mono- — None 341 R¹⁻⁸ Tri- R¹-R⁸ Quart-— None R¹-R⁸ Mono- 342 R¹⁻⁸ Tri- R¹-R⁸ Mono- R¹-R⁸ Quart- — None 343R¹⁻⁸ Tri- — None R¹-R⁸ Quart- R¹-R⁸ Mono- 344 R¹⁻⁸ Tri- R¹-R⁸ Mono- —None R¹-R⁸ Quart- 345 R¹⁻⁸ Tri- — None R¹-R⁸ Mono- R¹-R⁸ Quart- 346 R¹⁻⁸Tri- R¹⁻⁸ Pent- — None — None 347 R¹⁻⁸ Tri- — None R¹-R⁸ Pent- — None348 R¹⁻⁸ Tri- — None — None R1-R⁸ Pent- 348 R¹⁻⁸ Quart- R¹-R⁸ Mono- —None — None 349 R¹⁻⁸ Quart- — None R¹-R⁸ Mono- — None 350 R¹⁻⁸ Quart- —None — None R¹-R⁸ Mono- 351 R¹⁻⁸ Quart- R¹-R⁸ Mono- R¹-R⁸ Mono- — None352 R¹⁻⁸ Quart- R¹-R⁸ Mono- — None R¹-R⁸ Mono- 353 R¹⁻⁸ Quart- — NoneR¹-R⁸ Mono- R¹-R⁸ Mono- 354 R¹⁻⁸ Quart- R¹-R⁸ Mono- R¹-R⁸ Mono- R¹-R⁸Mono- 355 R¹⁻⁸ Quart- R¹⁻⁸ Di- — None — None 356 R¹⁻⁸ Quart- — NoneR¹-R⁸ Di- — None 357 R¹⁻⁸ Quart- — None — None R1-R⁸ Di- 358 R¹⁻⁸ Quart-R¹-R⁸ Di- R¹-R⁸ Mono- — None 359 R¹⁻⁸ Quart- R¹-R⁸ Di- — None R¹-R⁸Mono- 360 R¹⁻⁸ Quart- R³, R⁶ Di- R¹-R⁸ Mono- R¹-R⁸ Mono- 361 R¹⁻⁸ Quart-R¹-R⁸ Mono- R¹-R⁸ Di- — None 362 R¹⁻⁸ Quart- — None R¹-R⁸ Di- R¹-R⁸Mono- 363 R¹⁻⁸ Quart- R¹-R⁸ Mono- R¹-R⁸ Di- R¹-R⁸ Mono- 364 R¹⁻⁸ Quart-R¹-R⁸ Mono- — None R¹-R⁸ Di- 365 R¹⁻⁸ Quart- — None R¹-R⁸ Mono- R¹-R⁸Di- 366 R¹⁻⁸ Quart- R¹-R⁸ Mono- R¹-R⁸ Mono- R¹-R⁸ Di- 367 R¹⁻⁸ Quart-R¹-R⁸ Di- R¹-R⁸ Di- — None 368 R¹⁻⁸ Quart- R¹-R⁸ Di- — None R¹-R⁸ Di-369 R¹⁻⁸ Quart- — None R¹-R⁸ Di- R¹-R⁸ Di- 370 R¹⁻⁸ Quart- R¹⁻⁸ Tri- —None — None 371 R¹⁻⁸ Quart- — None R¹-R⁸ Tri — None 372 R¹⁻⁸ Quart- —None — None R1-R⁸ Tri 373 R¹⁻⁸ Quart- R¹-R⁸ Tri- R¹-R⁸ Mono- — None 374R¹⁻⁸ Quart- R¹-R⁸ Tri- — None R¹-R⁸ Mono- 375 R¹⁻⁸ Quart- R¹-R⁸ Mono-R¹-R⁸ Tri- — None 376 R¹⁻⁸ Quart- — None R¹-R⁸ Tri- R¹-R⁸ Mono- 377 R¹⁻⁸Quart- R¹-R⁸ Mono- — None R¹-R⁸ Tri- 378 R¹⁻⁸ Quart- — None R¹-R⁸ Mono-R¹-R⁸ Tri- 379 R¹⁻⁸ Quart- R¹⁻⁸ Quart- — None — None 380 R¹⁻⁸ Quart- —None R¹-R⁸ Quart- — None 381 R¹⁻⁸ Quart- — None — None R1-R⁸ Quart- 382R¹⁻⁸ Penta- R¹-R⁸ Mono- — None — None 383 R¹⁻⁸ Penta- — None R¹-R⁸ Mono-— None 384 R¹⁻⁸ Penta- — None — None R¹-R⁸ Mono- 385 R¹⁻⁸ Penta- R¹-R⁸Mono- R¹-R⁸ Mono- — None 386 R¹⁻⁸ Penta- R¹-R⁸ Mono- — None R¹-R⁸ Mono-387 R¹⁻⁸ Penta- — None R¹-R⁸ Mono- R¹-R⁸ Mono- 388 R¹⁻⁸ Penta- R¹-R⁸Mono- R¹-R⁸ Mono- R¹-R⁸ Mono- 389 R¹⁻⁸ Penta- R¹⁻⁸ Di- — None — None 390R¹⁻⁸ Penta- — None R¹-R⁸ Di- — None 391 R¹⁻⁸ Penta- — None — None R1-R⁸Di- 392 R¹⁻⁸ Penta- R¹-R⁸ Di- R¹-R⁸ Mono- — None 393 R¹⁻⁸ Penta- R¹-R⁸Di- — None R¹-R⁸ Mono- 394 R¹⁻⁸ Penta- R¹-R⁸ Mono- R¹-R⁸ Di- — None 395R¹⁻⁸ Penta- — None R¹-R⁸ Di- R¹-R⁸ Mono- 396 R¹⁻⁸ Penta- R¹-R⁸ Mono- —None R¹-R⁸ Di- 397 R¹⁻⁸ Penta- — None R¹-R⁸ Mono- R¹-R⁸ Di- 398 R¹⁻⁸Penta- R¹⁻⁸ Tri- — None — None 399 R¹⁻⁸ Penta- — None R¹-R⁸ Tri — None400 R¹⁻⁸ Penta- — None — None R1-R⁸ Tri

Particularly preferred anthraquinone and anthrahydroquinone derivativesare molecules with ID No. 87-89, 92-93, 96, 98-103, 107-110, 112, 118,211-212, 215, 217-230, 232, 234, 236-238, 241, 249, 263-264, 294-295,298, 300-310, 312, 314, 316, 318-319, 328, and 337-338.

The term “alkoxy” as used in tables 1-3 refers to C₁₋₆ alkoxy,preferably methoxy.

Further preferred redox active compounds according to General Formula(1), (2) and (3) are shown in FIG. 3.

In some embodiments, redox active compounds in the first and/or secondcomposition of the inventive combination according to general formula(1) are not selected from one or more of the following:

In some embodiments, redox active compounds in the first and/or secondcomposition of the inventive combination according to general formula(2) are not selected from one or more compounds according to thefollowing structural formulas:

In some embodiments, redox active compounds in the first and/or secondcomposition of the inventive combination according to general formula(3) are not selected from one or more compounds according to thefollowing structural formulas:

As discussed in greater detail elsewhere herein, redox activecompositions of the inventive combination may each comprise at least oneredox active compound as disclosed herein (optionally in both reducedand oxidized forms), or a mixture of several of these redox activecompounds. Particularly envisaged herein are mixtures of compounds whichdiffer (only) in their substitution pattern, especially theirsulfonation pattern. In other words, redox active compositions of theinventive combination may comprise or essentially consist of redoxactive distinctly sulfonated redox active compounds, i.e. whereindifferent residues “R” in the Formulas (1)-(3) represent —SO₃H groups,and wherein the compounds may preferably otherwise be characterized bythe same chemical structure.

3.4 Quinone Compounds: Preparation

The redox active compounds comprised by the inventive combination may beobtained from a variety of sources. For instance, said compounds mayadvantageously be prepared from lignin, as inter alia described inWO2017/174207 or WO2017/174206, which are incorporated by reference intheir entirety herein. Alternatively, said compounds may be obtainedfrom crude oil, coal or pure organic substances. The starting material(be it lignin, crude oil, coal or pure organic substances) is typicallysubjected to appropriate reaction conditions to obtain aromaticprecursor compounds, which may be processed to yield quinone compoundsthat are subjected to further substitution reactions to introduce thedesired substituents, such as —SO₃H, —OH or methoxy groups.

In general, sulfonation may be carried out in the presence ofconcentrated aqueous sulfuric acid. Alternatively, sulfur trioxide maybe mixed with inert gas, such as air, N₂ and/or CO₂, or complexed with acomplexing agent such as pyridine, dioxane, (CH₃)₃N or DMF. Typically,sulfonation is preferably performed at higher temperatures due toincreased resulting yields. Therein, an increased temperature isunderstood to be at least 50° C., preferably 100° C. However, thetemperature shall preferably not decompose the modified compound bypyrolysis. Accordingly, the temperature should preferably be lower than200° C. Separation of the resulting sulfonated compound(s) maysubsequently be carried out, for example, by filtration or salting out.

Accordingly, in order to prepare the first and second redox activecompounds of the inventive composition, quinone compounds may besubjected to sulfonation by treatment with SO), either from oleum or SO₃gas. The reaction is preferably performed under atmospheric pressure orelevated pressure in concentrated sulfuric acid at a temperature of40-300° C., preferably 60-120° C. for benzohydroquinones and 160-180° C.for anthraquinones. The reaction is undergone within 1-6 hours,preferably 3 hours for benzoquinones and 4 hours for anthraquinones.

After the reaction, the concentrated sulfuric acid may preferably bepoured into water and partially neutralized. The preferred neutralizingagent is calcium hydroxide, the terminative sulfuric acid concentrationis 5-30%, preferably 10-20%. After partially neutralizing the sulfuricacid, the precipitated sulfate may be filtered off. Subsequently, theresulting mixture may be directly concentrated, preferably under reducedpressure, to yield a solution of 0.4-1.5 mol/L active material and10-40% sulfuric acid. Alternatively, the solution may be completelyneutralized either with the same or another neutralizing agent and thewater may be evaporated under reduced pressure. Additional sulfates thateventually precipitate are filtered off such that the productprecipitates. The remaining water is then evaporated and the solid isdried to yield a mixture of 30-90% sulfonated product mixed withsulfates. Either process typically yields a crude mixture of differentlysulfonated quinone compounds, which may optionally be directly used asfirst or second redox active quinones.

4 Redox Active Compositions

The first and second redox active composition of the inventivecombination preferably comprise different redox active compoundsaccording to General Formulas (1)-(3). In other words, the first andsecond redox active compound of the first and second redox activecomposition are preferably characterized by different General Formulas(1)-(3), respectively. For instance, the first and second redox activecomposition may preferably be provided in the following combinations:

TABLE 4 Combinations of redox active compounds and compositions 1^(st)redox active compound in 2^(nd) redox active compound in # 1^(st) redoxactive composition 2^(nd) redox active composition 1 General Formula(1): General Formula (1): Benzoquinone(s) Benzoquinone(s) 2 GeneralFormula (1): General Formula (2) Benzoquinone(s) Naphthoquinone(s) 3General Formula (1): General Formula (3) Benzoquinone(s)Anthraquinone(s) 4 General Formula (2): General Formula (2):Naphthoquinone(s) Naphthoquinone(s) 5 General Formula (2): GeneralFormula (3): Naphthoquinone(s) Anthraquinone(s) 6 General Formula (2):General Formula (1): Naphthoquinone(s) Benzoquinone(s) 7 General Formula(3): General Formula (3): Anthraquinone(s) Anthraquinone(s) 8 GeneralFormula (3): General Formula (1): Anthraquinone(s) Benzoquinone(s) 9General Formula (3): General Formula (2) Anthraquinone(s)Naphthoquinone(s)

Therein, each redox active compound according to General Formula (1),(2) or (3) may optionally be present in its oxidized form (a) and/or itsreduced form (b).

In combinations according to #1, #4 and #7, i.e. wherein each redoxactive composition comprises benzo-, naphtho- and anthraquinonesaccording to General Formula (1), (2), or (3), respectively, as redoxactive compounds, said redox active compounds are preferably differentfrom one another. In particular, redox active compounds characterized bythe same General Formula (1), (2) or (3) preferably exhibit a differentsubstitution pattern.

Further, the first and/or second redox active composition may comprise amixture of redox active compounds, as described in greater detail below.

Preferably, the first and second redox active composition may beprovided in separate compartments. Said compartments may be containersor tanks, which are preferably electrically connected. More preferably,said compartments may be redox flow battery half-cells. Accordingly, thefirst and second redox active composition may preferably each beprovided in a half-cell of the same redox flow battery.

The first and second redox active compound contained in the first andsecond redox active composition (preferably used as posilyte andnegolyte in the inventive redox flow batteries, respectively) may beprovided in pure form, or may be dissolved or suspended in (a) suitablesolvent(s), optionally in combination with further additives.

Preferably, the first and second redox active composition (preferablyused as posilyte and negolyte in the inventive redox flow batteries) areprovided in liquid form, e.g. in pure liquid form or dissolved in asolvent.

4.1 Mixtures

The first and second redox active composition (preferably used asposilyte and negolyte in the inventive redox flow batteries,respectively) may comprise one or more different redox active compounds,preferably quinone compounds, as disclosed herein. In other words, thefirst and/or the second redox active composition may comprise 1 or amixture of 2, 3, 4, 5 or more different redox active compounds,preferably quinone compounds, as disclosed herein, and optionallysolvents and/or further additives. The term “redox active compositioncomprising at least one redox active compound, or a mixture thereof”thus refers to compositions comprising or (essentially) consisting of 1or 2, 3, 4, 5 or more different redox active compound(s).

In this context, the term “essentially consisting of” means acomposition comprising 1 or 2, 3, 4, 5 or more different redox activecompound(s) and a minor amount of by-products, impurities orcontaminants which preferably constitute less than 10 wt %, morepreferably less than 5 wt % of the overall composition by dry contentmass. The term “consisting” is understood as referring to a compositionwhich is exclusively composed of 1 or 2, 3, 4, 5 or more different redoxactive compound(s), without any by-products, impurities, contaminants orfurther additives. In other words, a redox active composition“consisting of” a redox active compound means a composition which iscomposed of a 100% pure compound.

When referring to a mixture of “different” quinone compounds, preferablymixtures of quinone compounds sharing a basic chemical structureaccording to either General Formula (1), (2) or (3) are meant. Thesequinone compounds may preferably differ in their substitution patterns.

For instance, the first redox active composition of the presentinvention may preferably comprise or essentially consist of 1 or 2, 3,4, 5 or more differently substituted benzoquinone compounds according toGeneral Formula (1). The second redox active composition of the presentinvention may preferably comprise or essentially consist of 1 or 2, 3,4, 5 or more differently substituted anthraquinone compounds accordingto General Formula (3).

Alternatively, it may also be conceivable to provide mixtures of quinonecompounds which differ in their basic chemical structure as representedby General Formula (1), (2) or (3).

Accordingly, redox active compounds of a “mixture” may be characterizedby the same or different General Formulas (1), (2) or (3). Said redoxactive compounds are typically different from one another. Inparticular, said redox active compounds present as a mixture in thefirst and/or second redox active composition preferably exhibit adifferent substitution pattern. For instance, the first and/or secondredox active composition may comprise a mixture of differentlysubstituted benzoquinones.

Combinations of redox active compositions #1 and #3 according to table 4may be particularly preferred. I.e., preferred combinations according tothe invention may comprise: a first redox active composition comprisinga benzoquinone according to General Formula (1), or a mixture ofdifferently substituted benzoquinones according to General Formula (1),as a first redox active compound; and a second redox active compositioncomprising a benzoquinone according to General Formula (2), or a mixtureof differently substituted benzoquinones according to General Formula(2), as a second redox active compound; or

a first redox active composition comprising a benzoquinone according toGeneral Formula (1), or a mixture of differently substitutedbenzoquinones according to General Formula (1), as a first redox activecompound; and an anthraquinone according to General Formula (3), or amixture of differently anthraquinones according to General Formula (3),as a second redox active compound.

Mixtures of redox active compounds preferably have the advantage thatcompounds, which are more expensive and/or difficult to produce, can bemixed with compounds, which are less expensive and/or difficult toproduce, but retain or even excel the desired redox properties of thesingle redox active compound (cf. Example 1).

4.2 Additional Redox Active Compounds

It may be preferred that the inventive combination includes redox activecompositions comprising redox active quinone compounds only.

However, it may also be conceivable to provide redox active compositionscomprising additional, non-quinone redox active compounds. Inalternative embodiments, the redox flow battery of the inventionincludes a first or second redox active composition as described hereinas a posilyte or negolyte, and the respective electrolyte of the counterelectrode, which preferably does not contain a quinone compoundaccording to General Formulas (1), (2) or (3) as disclosed herein.

For instance, additional redox active compounds may be selected from ametal or metal oxide such as vanadium, iron, chromium, cobalt, nickel,copper, lead, manganese, titanium, zinc or oxides thereof; a metal salt;a metal-ligand coordination compound such as hexacyanoiron complexes,e.g. ferrocyanide; bromine; chlorine; iodine; oxygen; an organic dyesuch as indigo carmine, viologen, methyl viologen or benzylviologen; anorganic compound such as tetrazole, diaryl ketone, dipyridyl ketone,dialkoxy benzene, phenothiazine, catechol, catechol ether, catecholphenylborate ester, and in particular tetrafluorocatechol,5-mercapto-1-methyltetrazoledi-(2-pyridyl)-ketone,2,5-di-tert-butyl-1,4-bis(2-methoxyethoxy)benzene (DBBB),2,5-di-tert-butyl-1,4-dimethoxybenzene,2,5-di-tert-butyl-1,4-bis(2,2,2-trifluoroethoxy)benzene, or5,6,7,8-tetrafluoro-2,3-dihydrobenzodioxine; and salts or mixturesthereof.

It may however be preferred that the first and/or second redox activecomposition does not comprise any additional non-quinone redox activecompound compound.

4.3 Solvents

The first and second redox active composition may preferably furthercomprise at least one solvent. Each composition may comprise a singlesolvent or a combination of two or more solvents. The solvent orsolvents of the first and second redox active composition may be thesame or may be different from each other.

In case the redox active compound of one or both of the first or secondredox active composition is a liquid material, it may also beconceivable that the solvent is omitted.

Solvents known in the battery art include, for example, organiccarbonates (e.g., ethylene carbonate (E), propylene carbonate (PC),dimethyl carbonate (DMC), diethyl carbonate (DEQ, ethyl methyl carbonate(EMC), and the like, or mixtures thereof), ethers (e.g., diethyl ether,tetrahydrofuran, 2-methyl tetrahydrofuran, dimethoxyethane, and 1,3dioxolane), esters (e.g., methyl formate, gamma-butyrolactone, andmethyl acetate), and nitriles (e.g., acetonitrile).

Particularly suitable solvents for dissolving or suspending the redoxactive compounds of the first and second redox active composition(preferably used as posilyte and negolyte in the inventive redox flowbatteries, respectively) include, without limitation, water, ionicliquids, methanol, ethanol, propanol, isopropanol, acetonitrile,acetone, dimethylsulfoxide, glycol, carbonates such aspropylenecarbonate, ethylene carbonate, propylene carbonate, dimethylcarbonate, diethyl carbonate, methyl ethyl carbonate, propylenecarbonate; polyethers such as dimethoxyethane; gamma-butyrolactonetetrahydrofuran; dioxolane; sulfolane; dimethylformamide;diethylformamide; CO₂ and supercritical CO₂; or a mixture thereof.

Preferably, the first and second redox active composition (preferablyused as posilyte and negolyte in the inventive redox flow batteries,respectively) may be provided as an aqueous solution. To that end, theredox active compound(s) may be dissolved or suspended in an aqueoussolvent system.

The term “aqueous solvent system” refers to a solvent system comprisingpreferably at least about 20% by weight of water, relative to totalweight of the solvent (which may optionally comprise further solvents,or additives as described in greater detail below).

The term “aqueous solvent system” thus includes solvents comprising atleast about 40%, at least about 50 wt %, at least about 60 wt %, atleast about 70 wt %, at least about 75 wt %, at least about 80%, atleast about 85 wt %, at least about 90 wt %, at least about 95 wt %, orat least about 98 wt % water, relative to the total solvent. For thepurpose of this calculation, any co-solvents are included in the weightof the solvent but any type of redox active compound, buffer, or othersupporting electrolyte is not considered a solvent, even if such speciesis a liquid. When a co-solvent is present, the co-solvent may besoluble, miscible, or partially miscible with water. Sometimes, theaqueous solvent may consist essentially of water, and be substantiallyfree or entirely free of any additives. The solvent system may be atleast about 90 wt %, at least about 95 wt %, or at least about 98 wt %water, or may be free of any additives.

In particular, the “aqueous solvent system” may include solventscomprising at least 40%, at least 50 wt %, at least 60 wt %, at least 70wt %, at least 75 wt %, at least 80%, at least 85 wt %, at least 90 wt%, at least 95 wt %, or at least 98 wt % water, relative to the totalsolvent.

Preferably, the first and second redox active composition may beprovided an a solution of 5-37 wt % sulphuric acid in water, morepreferably 10-30 wt %, even more preferably 15-25 wt % or and mostpreferably of 20 wt % sulphuric acid in water.

The first and second redox active composition of the inventivecombination are thus preferably provided as aqueous solutions. Whenapplied in redox flow batteries according to the invention, H⁺ ionsshuttle between the aqueous first and second redox active compositionused as posilyte and the negolyte, respectively, through a separator tobalance charges that develop during the oxidation and reduction of theredox active compounds.

It is, however, in principle also conceivable to provide the first andsecond redox active composition in non-aqueous solvent systems. In suchcases, additional electrolyte salt components may be added to thecompositions, and the cation component of said electrolyte salt maytravel between the compositions of the inventive combination to balanceout charges. The electrolyte salt components of the first and secondredox reactive composition may be any electrochemically stable salt.Said compositions may include a single salt or a combination of two ormore salts. The cation component of the electrolyte salt can be anymonovalent (e.g., Li⁺, Na⁺, Ag⁺, Cu⁺, NH₄ ⁺, and the like) ormultivalent cation (e.g., Mg²⁺, Ca²⁺, Cu²⁺, Zn²⁺, and the like). Thecation may comprise an alkali metal ion, such as lithium or sodium; analkaline earth metal ion, such as magnesium or calcium; and/or anorganic cation, such as tetraalkyl ammonium ions. The anionic componentof the electrolyte salts can be any anion suitable for use innon-aqueous electrolytes for lithium or sodium ion-type batteries. Somenon-limiting examples of suitable anionic components of the alkali metalsalts include tetrafluoroborate ion (BF₄ ⁻); hexafluorophosphate ion(PF₆ ⁻); perchlorate ion (ClO₄ ⁻); hexafluoroarsenate ion (AsF₆ ⁻);trifluoromethanesulfonate (“triflate” or CF₃SO₃ ⁻) ion;bis(perfluoroethanesulfonyl)imide (BETI) ion (N(SO₂CF₂CF₃)²⁻);bis(oxalato)borate (BOB) ion (B(C₂O₄)²⁻); halogen-substituted borane(B₁₂X_(n)H_((12-n)) ²⁻; X=halogen) ions; andbis(trifluoromethanesulfonyl)imide (TFSI) ion (N(SO₂CF₃)₂ ⁻). Theelectrolyte salts of the first and second redox active composition canbe different materials or can be composed of the same material ormaterials. Non-limiting examples of some electrolyte salts include,e.g., LiBF₄, LiPF₆, lithium triflate, NaBF₄, NaPF₆, and sodium triflate.

4.4 Concentration

The first and second redox active compound may be present in the firstand second redox active composition (preferably used as posilyte andnegolyte in the inventive redox flow batteries, respectively) in anysuitable amount.

The concentration of the redox active compound in the respectivecomposition is typically given as the total of the concentration of allredox active compounds present in the redox active composition. Theredox active composition may comprise a single redox active compound(e.g., quinone compound characterized by General Formula (1)).Alternatively, the redox active composition may comprise multiple redoxactive compounds (e.g., quinone compounds characterized by GeneralFormula (1) and (2), or differently substituted quinone compoundscharacterized by Formula (1.1) and (1.2)). The number of types of redoxactive compounds is not limited, and not all compounds need be a quinonecompound. However, it is typically preferred that all redox activecompounds contained within the redox active composition of the inventivecombination are redox active quinone compounds characterized by GeneralFormulas (1), (2) or (3).

The concentration of the first redox active compound in the first redoxactive composition (preferably used as posilyte in the inventive redoxflow batteries) may be between about 0.3 M and about 12 M. Preferably,the concentration of the first redox active compound in the first redoxactive composition may be at least about 0.3 M, at least about 0.5 M, atleast about 1 M, at least about 2 M, at least about 4 M, or at leastabout 6 M. Specifically, the concentration of the first redox activecompound in the first redox active composition may be between about 0.5M and about 2 M, between about 2 M and about 4 M, between about 4 M andabout 6 M, or between about 6 M and about 10 M.

In particular, the concentration of the first redox active compound inthe first redox active composition (preferably used as posilyte in theinventive redox flow batteries) may be between 0.3 M and 12 M.Preferably, the concentration of the first redox active compound in thefirst redox active composition may be at least 0.3 M, at least 0.5 M, atleast 1 M, at least 2 M, at least 4 M, or at least 6 M. Specifically,the concentration of the first redox active compound in the first redoxactive composition may be between 0.5 M and 2 M, between 2 M and 4 M,between 4 M and 6 M, or between 6 M and 10 M.

The concentration of the second redox active compound in the secondredox active composition (preferably used as a negolyte in the inventiveredox flow batteries) may be between about 0.3 M and about 12 M.Preferably, the concentration of the second redox active compound in thesecond redox active composition may be at least about 0.3 M, at leastabout 0.5 M, at least about 1 M, at least about 2 M, at least about 4 M,or at least about 6 M. Specifically, the concentration of the secondredox active compound in the second redox active composition may bebetween about 0.5 M and about 2 M, between about 2 M and about 4 M,between about 4 M and about 6 M, or between about 6 M and about 10 M.

The concentration of the second redox active compound in the secondredox active composition (preferably used as a negolyte in the inventiveredox flow batteries) may be between 0.3 M and 12 M. Preferably, theconcentration of the redox active compound in the redox activecomposition may be at least 0.3 M, at least 0.5 M, at least 1 M, atleast 2 M, at least 4 M, or at least 6 M. Specifically, theconcentration of the second redox active compound in the second redoxactive composition may be between 0.5 M and 2 M, between 2 M and 4 M,between 4 M and 6 M, or between 6 M and 10 M. The introduction ofsulfonyl groups may advantageously increase the solubility of theresulting sulfonated redox active (quinone) compound in water.Preferably, the redox active compounds of the first and second redoxactive composition are water-soluble. Preferably, the redox active(quinone) compounds of said compositions are water-soluble inconcentrations of at least 0.3 M, preferably at least 0.6 M, morepreferably at least 1.0 M at 25° C.

4.5 Additives

The first and/or second redox active composition (preferably used asposilyte and negolyte in the inventive redox flow batteries,respectively) of the inventive combination may optionally comprisefurther additives.

Preferably, the first and/or second redox active composition of theinventive combination may comprise co-solvents; buffering agents;emulsifying agents; further redox active compounds; supportingelectrolytes; ionic liquids; acids; bases; viscosity modifiers; wettingagents; stabilizers; salts; or combinations thereof.

Buffers may be selected from citrates, phosphates, borates, carbonates,silicates, carboxylates, sulfonates, alkoxides, trisaminomethane (Tris),4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid (HEPES),piperazine-N,N′-bis(ethanesulfonic acid) (PIPES), ammonium salts,pyridinium salts, and combinations thereof. Acids may be selected fromHCl, H₂SO₄, HClO₄, H₃PO₄, or HNO₃, or a mixture thereof. Bases may beselected from LiOH, NaOH, KOH, LiCl, NaCl, KCl, or a mixture thereof.Salts may be selected from those comprising monovalent cations (e.g., HLi Na⁺, K⁺, NH₄ ⁺, Cu⁺) or multivalent cations (e.g., Mg²⁺, Ca²⁺, Cu²⁺,Zn²⁺) or both, or those derived from alkali metals, alkaline metals ortransition metals, and any suitable anion (e.g., hydroxide, sulfate,nitrate, phosphate, carbonate, perchlorate, or borate) or anycombination or mixture thereof. Any combination of the aforementionedadditives is also envisaged herein for preparing the first and secondredox active composition of the inventive combination.

4.6 pH

The first and/or second redox active composition (preferably used asposilyte and negolyte in the inventive redox flow batteries,respectively) may be characterized as having a pH of between >0 andabout 14 or between about 0 and about 14.

The pH of the first and second redox active composition of the inventivecombination may be equal or substantially similar; or the pH of the twocomposition may differ by a value in the range of about 0.1 to about 2pH units, about 1 to about 10 pH units, about 5 to about 12 pH units,about 1 to about 5 pH units, about 0.1 to about 1.5 pH units, about 0.1to about 1 pH units, or about 0.1 to about 0.5 pH units. In thiscontext, the term “substantially similar,” without furtherqualification, is intended to connote that the difference in pH betweenthe two compositions is about 1 pH unit or less, such as about 0.4 orless, about 0.3 or less, about 0.2 or less, or about 0.1 pH units orless.

Preferably, the first and/or the second redox active composition mayhave a pH between about 7 and about 14, more preferably between about 9to about 14, most preferably between about 10 and about 12, or betweenabout 12 and about 14.

In particular, the pH of the two composition may differ by a value inthe range of 0.1 to 2 pH units, 1 to 10 pH units, 5 to 12 pH units, 1 to5 pH units, 0.1 to 1.5 pH units, 0.1 to 1 pH units, or 0.1 to 0.5 pHunits. Preferably, the first and/or the second redox active compositionmay have a pH between 7 and 14, more preferably between 9 to 14, mostpreferably between 10 and 12, or between 12 and 14.

The pH of the first and/or second redox active composition may bemaintained by a buffer, preferably a buffer as described in the section“additives” herein.

4.7 Reduction Potential

The redox active compounds of the inventive combination may preferablybe chosen to achieve a difference in standard reduction potential (or“standard cell potential”, E_(cell) ⁰) between the redox reactions ofthe first and second redox active compound which enables their use asposilyte or negolyte, respectively, in a redox flow battery as describedherein.

To that end, the redox active compounds may be chosen to achieve astandard cell potential of at least 0.0 V, preferably at least +0.5 V,more preferably at least +0.8 V, even more preferably at least +1.0 V,and most preferably of at least +1.5 V against SHE, typically between+0.5 and +1.5 V, preferably between +0.8 and +1.2 V against SHE.

The standard cell potential (E⁰ _(cell)) is the difference in thestandard reduction potentials (against the standard hydrogen electrode(SHE)) of the two half-cell reactions at the cathode and anode:

E _(cell) ⁰ e=E _(cat) ⁰ −E _(an) ⁰  eq. 1

(ΔE⁰ _(cell)=standard cell potential, E⁰ _(cat): standard reductionpotential for the reduction half-reaction occurring at the cathode, E⁰_(an): standard reduction potential for the oxidation half-reactionoccurring at the anode).

The Nernst Equation (eq. 2) enables the determination of the cellpotential under non-standard conditions. It relates the measured cellpotential to the reaction quotient and allows the accurate determinationof equilibrium constants (including solubility constants).

$\begin{matrix}{E_{cell} = {E_{cell}^{0} - {\frac{RT}{n\; F}\ln \; Q}}} & {{eq}.\mspace{14mu} 2}\end{matrix}$

(E_(cell)=cell potential under non-standard conditions, n=number ofelectrons transferred in the reaction, F=Faraday constant (96,500C/mol), T=Temperature and Q=reaction quotient of the redox reaction).

The standard reduction potential (E_(cat) ⁰, E_(an) ⁰) of each redoxactive compound is characteristic for each compound and its substitutionpattern and is inter alia related to the electronic energy of themolecular orbitals. Preferably, the addition of —SO₃H groups mayincrease the standard reduction potential of the resulting compound,which is consistent with the lowering of molecular orbital energies byelectron-withdrawing groups.

To that end, the first redox active compound of the inventivecombination may preferably exhibit a standard reduction potential (E⁰)of at least about 0.0 V, more preferably of at least about +0.5 V, evenmore preferably of at least about +0.6 V, most preferably of at leastabout +0.7 V or more against SHE. The second redox active compound ofthe inventive combination may preferably exhibit a standard reductionpotential (E⁰) of about +0.3 V or less, more preferably of about +0.1 Vor less, even more preferably of about 0.0 V or less, about −0.5 V orless, about −0.6V or less, about −1.0V or less or about −1.2 V or less.

In particular, the first redox active compound of the inventivecombination may preferably exhibit a standard reduction potential (E⁰)of at least 0.0 V, more preferably of at least +0.5 V, even morepreferably of at least +0.6 V, most preferably of at least +0.7 V ormore against SHE. The second redox active compound of the inventivecombination may preferably exhibit a standard reduction potential (E⁰)of +0.3 V or less, more preferably of +0.1 V or less, even morepreferably of 0.0 V or less, −0.5 V or less, −0.6V or less, −1.0V orless or −1.2 V or less.

Preferably, the first redox active compound may have a standardreduction potential that is at least 0.3 V higher than the standardreduction potential of the second redox active compound.

Redox active (quinone) compounds featuring more positive (less negative)standard reduction potentials (E⁰) and redox active compositionscomprising the same are particularly suitable as posilytes in redox flowbatteries of the invention. Redox active (quinone) compounds featuringmore negative (less positive) standard reduction potentials (E⁰) andredox active compositions comprising the same are particularly suitableas negolytes in redox flow batteries of the invention.

5. Redox Flow Battery

In a further aspect, the present invention provides a redox flow batterycomprising the inventive combination of first and second redox activecomposition.

5.1 Redox Flow Battery: Assembly

In particular, the present invention provides a redox flow batterycomprising a positive electrode contacting a first electrolyte (alsoreferred to herein as a “positive electrolyte”, “posilyte”), a negativeelectrode contacting a second electrolyte (also referred to herein as a“negative electrolyte”, “negolyte”), and a separator interposed betweenthe positive electrode and the negative electrode.

Preferably, the electrolytes are provided in liquid form, either in pureliquid form or dissolved in a solvent. The electrodes are preferably influid communication with the posilyte or negolyte, respectively.

Preferably, redox flow batteries according to the invention furthercomprise a positive electrode reservoir (“posilyte chamber”) comprisingthe positive electrode immersed within the positive electrodeelectrolyte, said positive electrode chamber forming the first redoxflow battery half-cell; and a negative electrode chamber (“negolytechamber”) comprising the negative electrode immersed within the negativeelectrode electrolyte, said negative electrode chamber forming thesecond redox flow battery half-cell.

Each chamber and its associated electrode and electrolyte thus definesits corresponding redox flow battery half-cell. Each electrolytepreferably flows through its corresponding half-cell flow so as tocontact the respective electrode disposed within the electrolyte, andthe separator. The electrochemical redox reactions of the electrolytesoccur within said half-cells.

The separator is preferably disposed between both electrodes and, thus,both half-cells, and thereby partitions both half-cells from each other.

Preferably, the first redox active composition of the inventivecombination may be used as the posilyte. Said posilyte is typicallyprovided in liquid form, and the positive electrode is preferablyimmersed in the liquid posilyte within the posilyte chamber.

The posilyte chamber is defined by a second housing or enclosure. Theposilyte chamber is preferably adapted to communicate with a firstpositive electrolyte reservoir (“posilyte reservoir”) and optionally asecond posilyte reservoir (e.g., via openings, valves, tubing, and thelike to connect the interior of the housing/enclosure with the interiorof the reservoirs). The first posilyte reservoir, the posilyte chamber,and optionally the second posilyte reservoir together define a posilytecirculation pathway. A pump is preferably operably positioned within theposilyte circulation pathway to facilitate circulation of the posilytewithin the posilyte circulation pathway over the positive electrode. Thepump may be positioned in any convenient location in the posilyte flowpathway (e.g., between the first posilyte reservoir and the posilytechamber, between the second posilyte reservoir and the posilyte chamber,or integral with a portion of the posilyte chamber or posilytereservoirs).

Preferably, the second redox active composition of the inventivecombination may be used as the negolyte. Said negolyte is typicallyprovided in liquid form, and the negative electrode is preferablyimmersed in the liquid negolyte within the negolyte chamber.

The negolyte chamber is defined by a first housing or enclosure. Thenegolyte chamber is preferably adapted to communicate with a firstnegative electrolyte reservoir (“negolyte reservoir”) and optionally asecond negolyte reservoir (e.g., via openings, valves, tubing, and thelike to connect the interior of the housing/enclosure with the interiorof the reservoirs). The first negolyte reservoir, the negolyte chamber,and optionally the second negolyte reservoir together define a negolytecirculation pathway. A pump is preferably operably positioned within thenegolyte circulation pathway to facilitate circulation of the negolytewithin the negative electrolyte circulation pathway and over thenegative electrode. The pump may be positioned in any convenientlocation in the negolyte flow pathway (e.g., between the first negolytereservoir and the negolyte chamber, between the second negolytereservoir and the negolyte chamber, or integral with a portion of thenegolyte chamber or negolyte reservoirs).

The posilyte and negolyte chambers are electrically connected to anelectrical power supply or load. They may be composed of any materialthat is preferably (electro-)chemically inert and suitable to retain therespective electrolytes.

The redox flow battery may further comprise at least one posilytereservoir and at least one negolyte reservoir, which function as storagetanks for the posilyte and negolyte, respectively.

The tank volume preferably determines the quantity of energy stored inthe system, which may be measured in kWh.

The circulation loops of the redox flow battery may comprise any valves,rigid or flexible tubes, pipes, bypass loops, manifolds, joints,openings, apertures, filters, pumps, gas inlets and outlets,pressurizing devices, pressure release features, pressure equalizingfeatures, flow features, or any other features suitable for systems forliquid and gas handling.

Pumps suitable for use in the redox flow batteries described hereininclude internal gear pumps, screw pumps, shuttle block pumps, flexiblevane pumps, sliding vane pumps, circumferential piston pumps, helicaltwisted root pumps, piston pumps, diaphragm pumps, peristaltic pumps,centrifugal pumps, and the like, which are well known in the liquidpumping art. The utility of a given pump will be dependent on thechemical resistance of the pump to the electrolyte components in contacttherewith (i.e., materials compatibility). Alternatively, theelectrolytes may be recirculated by any other method, e.g., stirring,convection, sonication, etc., which may obviate the need for pumps.

The redox flow battery may further comprise any controllers, sensors,meters, alarms, wires, circuits, switches, signal filters, computers,microprocessors, control software, power supplies, load banks, datarecording equipment, power conversion equipment, and other devicessuitable for operating a battery and optionally ensuring safe,autonomous, and efficient operation of the redox flow battery. Suchsystems and devices are known to those of ordinary skill in the art.

FIG. 1 schematically illustrates a redox flow battery that includes acombination of redox active compositions according to the invention.Redox flow battery 100 includes at least one redox flow battery cell 110composed of positive electrolyte chamber 120 and negative electrolytechamber 130. Positive electrolyte (posilyte) chamber 120 containspositive electrode 122 immersed in a posilyte 124, and negativeelectrolyte (negolyte) chamber 130 contains negative electrode 132immersed in negolyte 134. Several redox flow battery cells may becombined to form an electrochemical cell stack (not shown).

Separator 112 is interposed between posilyte chamber 124 and negolytechamber 134 and, and allows passage of cations (C⁺) back and forthbetween the posilyte and negolyte to balance out charges that formduring oxidation and reduction of materials within the electrolytes.Reduction occurs during discharge at the positive electrode andoxidation occurs during discharge at the negative electrode. Conversely,oxidation occurs during charging at the positive electrode and reductionoccurs during charging at the negative electrode.

Redox flow battery 100 further includes a positive electrode reservoir126 in fluid communication with the positive electrode 122. The positiveelectrode electrolyte 124 is stored in the positive electrode reservoir126 to charge and discharge the redox flow battery. The positiveelectrode electrolyte cycles through battery cell 110 from positiveelectrode reservoir 126 via the pumping action of pump 128. A negativeelectrode reservoir 136 is in fluid communication with the negativeelectrode 132. The negative electrode electrolyte 134 is stored in thenegative electrode reservoir 136 to charge and discharge the flowbattery. The negative electrode electrolyte cycles through battery cell110 from negative electrode reservoir 136 via the pumping action of pump138.

5.2 Redox Flow Battery: Function

A redox flow battery according to the invention may be both charged anddischarged. During charge, redox active compounds contained in theposilyte (preferably: the first redox active composition disclosedherein) undergo oxidation, and redox active compounds contained in thenegolyte (preferably: the second redox active composition disclosedherein) undergo reduction, whereas during discharge, redox activecompounds contained in the posilyte (preferably: the first redox activecomposition disclosed herein) undergo reduction, and redox activecompounds contained in the negolyte (preferably: the second redox activecomposition disclosed herein) undergo oxidation.

During charge of the redox flow battery, the reduction product H₂Q¹ ofthe at least one first redox active compound in the first redox activecomposition may be oxidized to a first oxidation product Q¹. At the sametime, the oxidation product Q² of the at least one second redox activecompound in the second redox active composition may be reduced to the afirst reduction product H₂Q².

During discharge of the redox flow battery, the oxidation product Q¹ ofthe at least one first redox active compound in the first redox activecomposition may be reduced to regenerate the first reduction productH₂Q¹. At the same time, the reduction product H₂Q² of the at least onesecond redox active compound of the second redox active composition mayoxidized to regenerate the second organic compound Q².

Charging of the inventive redox flow battery is typically accomplishedby applying an electric potential to the negative and positiveelectrodes, while simultaneously pumping the negolyte over the negativeelectrode, typically from the first negolyte reservoir to a secondnegolyte reservoir, and pumping the positive electrolyte over thepositive electrode, typically from the first posilyte reservoir to asecond posilyte reservoir. Cations flow across the separator to balancethe charges.

During charge, electrons move from the power supply to the negativeelectrode, where they are transferred to the redox active compoundscontained in the negolyte, converting those compounds to their reducedform(s). Electrons are also transferred from redox active compounds ofthe posilyte to the positive electrode, and to the power supply,converting those compounds to oxidized form(s). Ions (preferably H⁺)shuttle between the posilyte and negolyte to balance the charges thatdevelop as a result of oxidation and reduction of redox active compoundsin each electrolyte. The positive (higher) potential redox activecompound present in the posilyte is thereby oxidized, while the negative(lower) potential redox active compound present in the negolyte isreduced.

Accordingly, the inventive redox flow battery is preferably charged byapplying a potential difference across the first and second electrode,such that the first redox active compound is oxidized, and the secondredox active compound is reduced.

The oxidized and reduced redox active composition may be transported toand stored in posilyte and negolyte reservoirs, respectively. Thereby,energy can be stored by charging the battery from an energy source.

Discharging of the inventive redox flow battery is achieved by placingthe electrodes in a circuit (e.g., with a power grid) and reversing thedirection of electrolyte flow, with the stored reduced redox activecompound present in the negolyte (preferably: the second redox activecomposition) being pumped over the negative electrode typically backinto the first negolyte reservoir, and the stored oxidized positiveredox reactive compound present in the posilyte (preferably: the firstredox active composition) being pumped over the positive electrode backinto the first posilyte reservoir. Cations again flow across theion-permeable separator (in the opposite direction) to balance thecharges.

During discharge, electrons are transferred from the reduced form(s) ofthe redox active compound(s) contained in the negolyte to the negativeelectrode, and to the power supply, converting those reduced forms tooxidized form(s). Electrons also move from the power supply to thepositive electrode and are transferred to the oxidized form(s) of theredox active compound(s) contained in the posilyte, converting them toreduced form(s). Ions (preferably H⁺) shuttle between the posilyte andnegolyte to balance the charges that develop as a result of oxidationand reduction of redox active compounds in each electrolyte.

Accordingly, the inventive redox flow battery is preferably dischargedby applying a potential difference across the first and second electrodesuch that the first redox active compound is reduced, and the secondredox active compound is oxidized.

The energy stored in the system can thus be directly used to performwork or can be transferred back into the power grid during peak usageperiods to supplement the power supply. An AC/DC converter can be usedto facilitate transfer of energy to and from an AC power grid.

Referring to FIG. 1, posilyte 124 preferably comprises water and atleast one first redox active compound which is preferably a firstquinone compound (Q¹/H₂Q¹). The posilyte 124 flows over and contactspositive electrode 122. Preferably, the at least one quinone compound ofthe posilyte 124 may be characterized by General Formula (1). Thenegolyte 134 preferably comprises water and at least one second redoxactive compound which is preferably a second quinone compound (Q²/H₂Q²).The negolyte 134 flows over and contacts negative electrode 132. Inpreferred embodiments, the at least one quinone compound of the negolytemay be characterized by General Formula (3).

During charge, electrons are transferred from negative electrode 132 tonegolyte 133. The oxidized forms of the second quinone compound (Q²)accept these electrons and are reduced to their hydroquinone forms(H₂Q²). Electrons are transferred from posilyte 124 to positiveelectrode 122. The reduced hydroquinone forms (H₂Q¹) of the firstquinone compound donate electrons and are thereby oxidized to theirquinone forms (Q¹). H⁺ ions travel through the separator to balance outcharges. During discharge, the electrical flow is reversed and oxidizedquinone forms of the second redox active compound (Q²) and reducedhydroquinone forms of the first redox active compound (H₂Q¹) areregenerated at the negative and positive electrode, respectively.

The redox reactions of the inventive redox flow battery generallyproceed at temperatures of 100° C. or less; typically 25° C. or less.Generally, the redox reactions of the battery of the present inventionproceed at temperatures of between 10 and 100° C.

Oxygen may or may not be excluded from the battery of the presentinvention. The redox reactions may proceed within a closed system, e.g.under an inert atmosphere such as N₂, if necessary.

5.3 Electrolytes

The selection of redox flow battery electrolytes and electrode materialsis generally based on their electrochemical properties (e.g., stabilitywindow), physical properties (e.g., viscosity, vapor properties), safety(e.g., corrosiveness, toxicity), and cost.

As indicated previously, the posilyte and negolyte are preferablyselected based on their standard reduction potentials vs. SHE in orderto provide a redox flow battery with a high cell potential. The redoxactive compound(s) contained in the posilyte is preferably selected tohave a redox potential which is higher than that of the redox potentialof the redox active compound(s) contained the negative electrolyte. Byselecting a first and second redox active composition based on redoxactive compounds that are far apart in standard reduction potential, theredox flow battery cell potential can be maximized, e.g. to +1.2 V.

Preferably, the first redox active composition of the inventivecombination may comprise a redox active compound exhibiting a higher(more positive) standard reduction potential, and may be employed as theposilyte in the inventive redox flow battery. Preferably, the secondredox active composition of the inventive combination may comprise aredox active compound exhibiting a lower (more negative) standardreduction potential, and may be employed as the negolyte in theinventive redox flow battery. The inventive redox flow battery thuspreferably comprises the first and second redox active composition ofthe inventive combination as electrolytes, and is therefore an“all-organic” redox flow battery.

In the redox flow batteries according to the invention, the posilyte andnegolyte preferably each contain a redox active compound (or a mixtureof several redox active compounds) as disclosed herein. The redoxreactive compound (or mixture of compounds) contained within theposilyte usually exhibits a higher redox potential than the redoxreactive compound (or mixture of compounds) contained within thenegolyte. Preferably, the first redox active composition of theinventive combination is used as the posilyte, and the second redoxreactive composition of the inventive combination is used as thenegolyte. It is, however, generally also conceivable to reverse theroles of the first and second redox active compositions, or to combinethe first or second redox active composition as posilyte/negolyte withanother electrolyte as negolyte/posilyte.

In a further aspect, the invention thus relates to the use of thecombination of redox active compositions as redox flow batteryelectrolytes, wherein the first redox active composition is preferablyused as a positive electrode electrolyte, and the second redox activecomposition is preferably used as a negative electrode electrolyte.

It is generally also conceivable to use the first or second redox activecomposition disclosed herein independently from each other in redox flowbatteries, optionally in combination with other (non-quinone)electrolytes. Said other electrolytes may be selected from other organicredox active compounds. Alternatively, said other electrolytes may beselected from inorganic redox active compounds, thereby providing a“half-organic” redox flow battery.

For instance, such redox flow batteries may employ redox activecomposition comprising or consisting of a quinone compound as anelectrolyte in one half-cell of the battery, and another, optionallyinorganic, redox active compound, as an electrolyte in the otherhalf-cell of the battery. Said “other” redox active compound may beselected from halogen, transition metal ions, or metal ligandcoordination compounds, including bromine, chlorine, iodine, oxygen,vanadium, chromium, cobalt, iron, manganese, chromium, titanium, zinc,cobalt, nickel, copper, lead, or a salt or oxide thereof. Alternatively,said “other” redox active compound may be selected from organiccompounds (e.g. hexacyanoiron complexes, other quinone compounds, or anorganic dyes e.g. indigo carmine, viologen, methyl viologen orbenzylviologe or salts or mixtures thereof).

Preferably, the redox flow battery thus comprises a positive electrode;a first redox active composition as disclosed herein as a positiveelectrode electrolyte (“posolyte”), the positive electrode electrolytecontacting the positive electrode; a negative electrode; a second redoxactive composition as disclosed herein as a negative electrodeelectrolyte (“negolyte”), the negative electrode electrolyte contactingthe negative electrode; and a separator, preferably a membrane,interposed between the positive electrode and the negative electrode.The first and second redox active compounds are each preferably quinonecompounds according to General Formulas (1), (2) or (3), or mixturesthereof.

The posilyte may preferably correspond to the first redox activecomposition, and the negolyte may preferably correspond to the secondredox active composition of the inventive combination. It may, however,also be conceivable to use compounds disclosed in connection with thesecond redox active composition as posilytes, and compounds disclosed inconnection with the first redox active composition as negolytes.

The first redox active composition of the inventive combination may beused as the positive electrolyte. The second redox active composition ofthe inventive combination may be used as the negolyte. Accordingly, theposilyte may comprise at least one redox active compound, preferably aquinone compound, as characterized by any of General Formulas (1)-(3),preferably General Formulas (1) or (2), and more preferably GeneralFormula (1) as further defined elsewhere herein. Accordingly, thenegolyte may comprise at least one redox active compound, preferably aquinone compound, as characterized by any of General Formulas (1)-(3),preferably General Formulas (2) or (3), and more preferably GeneralFormula (3) as further defined elsewhere herein.

The first and second redox active composition of the inventive redoxflow battery used as posilyte and negolyte, respectively, may comprisethe same or different redox active compounds as described herein.Preferably, the posilyte and negolyte comprise different redox activecompounds.

The first and second redox active composition (preferably used asposilyte and negolyte, respectively, of the inventive redox flowbattery) may preferably comprise quinone compounds as disclosed herein,which are dissolved or suspended in aqueous solution. The concentrationof the quinone compound may range, for example, from 3 M to liquidquinone, e.g., 3-15 M. In addition to water, the compositions mayinclude co-solvents such as alcohols to increase the solubility of aparticular quinone, and optionally further additives. The compositionsmay comprise at least 10%, 20%, 30%, 40%, 50%, 60%, 70%, or 80% water,by mass. Alcohol or other co-solvents may be present in an amountrequired to result in a particular concentration of the quinonecompound(s). The pH of the aqueous solution may also be adjusted byaddition of acid or base, e.g., to aid in solubilizing the quinonecompound(s).

Preferred redox active compounds contained within the posilyte and/ornegolyte are disclosed in sections 3 and 4 herein.

Preferred solvents contained within the posilyte and/or negolyte aredisclosed in section 4.3 herein.

Preferred concentrations of the redox active compounds within theposilyte and/or negolyte are disclosed in section 4.4 herein.

Preferred additives contained within the posilyte and/or negolyte aredisclosed in section 4.2 and 4.5 herein.

Preferred pH values of the posilyte and/or negolyte are disclosed insection 4.6 herein.

Preferred reduction potentials of the redox active compounds containedwithin the posilyte and/or negolyte are disclosed in section 4.7 herein.

5.4 Electrodes

The inventive redox flow battery comprises a first (positive) and second(negative) electrode (cathode and anode, respectively).

The negative and positive electrodes of the inventive redox flow batteryprovide a surface for electrochemical reactions during charge anddischarge. As used herein, the terms “negative electrode” and “positiveelectrode” are electrodes defined with respect to one another, such thatthe negative electrode operates or is designed or intended to operate ata potential more negative than the positive electrode (and vice versa),independent of the actual potentials at which they operate, in bothcharging and discharging cycles. The negative electrode may or may notactually operate or be designed or intended to operate at a negativepotential relative to the reversible hydrogen electrode. The positiveelectrode is associated with the posilyte and the negative electrode isassociated with the negolyte, as described herein.

5.5 Separator

The inventive redox flow battery further includes a separator. Saidseparator typically separates the first from the second electrode, eachpreferably immersed in its corresponding electrolyte within one redoxflow battery half-cell. Said separator preferably (1) physicallyseparates the posilyte and negolyte, thereby preferably preventing orimpeding the mixing of posilyte and negolyte; (2) reduces or preventsshort circuits between the positive and negative electrodes, i.e. serveas an insulator between both electrodes; and (3) enables ion (typically,H⁺) transport between the positive and negative electrolyte chambers,thereby balancing electron transport during charge and discharge cycles.The electrons are primarily transported to and from an electrolytethrough the electrode contacting that electrolyte.

Suitable separator materials may be chosen by the skilled artisan fromseparator materials known in the art as long as they are(electro-)chemically inert and do not, for example, dissolve in thesolvent or electrolyte. Separators are preferably cation-permeable, .e.allow the passage of cations such as H⁺ (or alkali ions, such as sodiumor potassium), but is at least partially impermeable to the redox activecompounds.

Examples of ion-conductive separators include ion exchange membranes, inparticular cation exchanges membranes, e.g. NAFION® type ion exchangemembranes (sulfonated tetrafluoroethylene-basedfluoropolymer-copolymers), other preferably porous polymeric materialssuch as, for example, sulfonated poly(ether ether ketones),polysulfones, poly-styrene, polyethylene, polypropylene,ethylene-propylene copolymers, polyimides, polyphenylene, bi-phenylsulfone (BPSH), polyvinyldifluorides, or thermoplastics such aspolyetherketones or polyethersulfones, and the like; each of which canbe in the form of membranes, matrix-supported gels, sheets, films, orpanels. Other suitable materials include porous ceramics, porousinsulated metals, cation-conducting glasses, and zeolites.Alternatively, the separator may be selected from size exclusionmembranes, e.g., porous ultrafiltration or dialysis membranes with amolecular weight cut off of 100, 250, 500, or 1,000 Da. Alternatively,the separator may be an interface between immiscible liquids. In suchcase, a porous film, panel, or mesh might be included to aid inmaintaining separation between the liquids (e.g., as a physical supportor guide to aid in maintaining laminar flow at the interface. For sizeexclusion membranes, the required molecular weight cut off is determinedbased on the molecular weight of the redox active species compoundemployed. Porous physical separators may also be included, in caseswhere the passage of redox active species is tolerable. Such porousseparators may comprise or consist of high density polyethylene,polypropylene, polyvinylidene difluoride (PVDF), orpolytetrafluoroethylene (PTFE), optionally in combination with suitableinorganic fillers including silicon carbide matrix material, titaniumdioxide, silicon dioxide, zinc phosphide, and ceria. Separator comprisesmultiple components and/or materials are also envisaged. For instance,separators may comprise two or more layered membranes or a coatedmembrane.

Separators of the present invention may feature a thickness of about 500microns or less, about 300 microns or less, about 250 microns or less,about 200 microns or less, about 100 microns or less, about 75 micronsor less, about 50 microns or less, about 30 microns or less, about 25microns or less, about 20 microns or less, about 15 microns or less, orabout 10 microns or less, for example to about 5 microns.

Preferably, the inventive redox flow battery may comprise anion-permeable separator comprising or essentially consisting of a cationexchange membrane, optionally selected from a polymer membrane, morepreferably from a sulfonate containing fluoropolymer.

In the inventive redox flow batteries, the first and second housings orenclosures for the posilyte and negolyte chambers, respectively, areintegral with one another, and the separator is mounted as an internalpartition separating the both electrolyte chambers from each other.Alternatively, the first and second housings can be separate componentsthat include perforations or openings that contact the separator, suchthat cations can flow between the electrolyte chambers, optionally alongwith some of the solvent and or redox active compound, and the separatehousings are sealed, e.g. by gaskets, around the partition.

5.6 Redox Flow Battery: Advantages

Redox flow batteries deploying combinations of redox active compounds,preferably quinone compounds as defined herein, exhibit a number ofadvantages.

A particular advantage of redox flow batteries is the decoupling ofpower and energy. The energy capacity of such a system can be changedwithout changing the system power. For example, increasing the volume ofelectrolyte can add energy capacity without requiring any change to theelectrochemical stack. In contrast, in order to increase the energycapacity of a typical sealed battery (e.g., lithium ion) the size of theelectrochemical stack must be increased.

Current Density

The inventive redox flow batteries may be characterized in terms oftheir current density. The “current density” is the measurement ofelectric current (charge flow in amperes) per unit area of cross-section(cm²).

The current and, thus, the power (as the product of current andreduction potential) of any redox flow battery depends on the number ofelectrons involved in the redox reactions of its half-cells.

The redox active compounds contained in the redox active compositions ofthe inventive combination are preferably quinone compounds characterizedby General Formula (1), (2) or (3).

Such compounds preferably undergo two-electron redox reactions.Accordingly, the current of the redox flow battery of the presentinvention is preferably higher than most known redox flow batteries,which often rely on one electron transfer redox reactions.

Preferably, the current density of the inventive redox flow battery maybe at least 0.5 Amp/cm² of electrode area or more. As such, the redoxflow battery of the present invention can preferably support heavycurrent devices, and can preferably maintain a high current forrelatively long periods of time. Preferably, the voltage generated bythe redox flow battery of the present invention is higher than thevoltage generated by most known redox flow batteries.

Energy Density

The inventive redox flow batteries may be characterized in terms oftheir energy density. The “energy density” is the amount of energystored in a given system per unit volume.

The energy density of redox flow batteries may be limited by both thesolubility of the redox-active compounds employed, and the number ofelectrons transferred. As the employed first and second redox activecompound, preferably quinone compounds, of the invention are preferablyhighly soluble in water, and are capable of undergoing two-electrontransfers, the inventive redox flow battery preferably exhibits a highenergy density.

Redox flow batteries of the present invention may operate with an energydensity of, at least between about 10 Wh/L per side and about 20 Wh/Lper side, preferably between about 20 Wh/L per side and about 50 Wh/Lper side, most preferably between about 50 Wh/L per side and about 100Wh/L per side, In certain embodiments, the electrolyte-only energydensity is between about 5 and about 10 Wh/L, between about 10 and about20 Wh/L, between about 20 and about 40 Wh/L, or between 40 and 60 Wh/L.

Shelf-Life

Due to their stability towards redox cycling, the quinone compounds canundergo multiple cycles of reduction and oxidation reactions. Redox flowbatteries deploying quinone compounds as redox active compounds in theirpositive and/or negative electrolytes can therefore preferably berecharged repeatedly, and still achieve up to at least 90% of itsinitial reduction potential, preferably at least 95% of its initialreduction potential, more preferably up to approximately 100% of itsinitial reduction potential.

Accordingly, the redox flow battery of the present invention preferablyexhibits a long and predictable shelf-life relative to knownrechargeable redox flow-through batteries.

Energy Efficiency

The inventive redox flow batteries may be characterized in terms oftheir energy efficiency. The term “energy efficiency” or “round tripefficiency” refers to the ratio of total energy obtained from dischargeto the energy provided during charge in a cycle.

The energy efficiency may be calculated as the product of the voltageefficiency and current efficiency, which are defined herein. The redoxflow battery's round trip efficiency may be at least about 70%, at leastabout 75%, at least about 80%, at least about 85%, or at least about90%. Preferably, the round trip current efficiency is at least about95%, at least about 98%, at least about 99%, at least about 99.5%, atleast about 99.9% or at least about 99.99%.

In particular, the redox flow battery's round trip efficiency may be atleast 70%, at least 75%, at least 80%, at least 85%, or at least 90%.Preferably, the round trip current efficiency is at least 95%, at least98%, at least 99%, at least 99.5%, at least 99.9% or at least 99.99%.

Capacity

Preferably, and in particular in all-organic quinone compound-basedredox flow batteries comprising the first and second redox activecomposition as posilyte and negolyte, respectively, the same number ofelectrons, and thus the same amount of charge is involved in theoxidation and reduction reactions occurring at each half-cell of thebattery. The inventive redox flow battery is therefore preferablycharge-balanced. Typically, the inventive redox flow battery may be setup as a closed system, and is therefore mass-balanced as well.

Due to the charge- and mass-balanced nature of the inventive redox flowbattery, it may be scaled up easily and efficiently.

The redox flow battery's energy capacity is a function of theelectrolyte volume. By increasing the volume of the posilyte andnegolyte, the energy capacity of the inventive redox flow battery can beenhanced.

Preferably, the energy capacity of the redox flow battery of the presentinvention may be at least 0.01 Amp hour per cm³ of electrolyte, morepreferably at least 0.1 Amp hour per cm³ of electrolyte, and mostpreferably 1 Amp hour per cm³ of electrolyte.

Safety

Furthermore, since the redox flow battery of the present invention ispreferably charge- and mass-balanced and operates as a closed system, itposes a very low fire, explosion or toxicity risk compared to knownbatteries, in particular compared to known batteries having a similarenergy capacity.

In contrast to known batteries, the redox flow battery of the presentinvention preferably does not include any metals or corrosiveelectrolytes and is therefore eco-friendly and non-toxic.

5.7 Redox Flow Battery Stacks

In some cases, a user may desire to provide higher charge or dischargevoltages than available from a single redox flow battery. In such cases,several redox flow batteries of the invention may be connected inseries, thereby forming a redox flow battery cell stack. In such cellstacks, the voltage of each redox flow battery cell is additive.

An electrically conductive, but non-porous material (e.g., a bipolarplate) may be employed to connect adjacent redox flow battery cells in abipolar stack, which allows for electron transport but prevents fluid orgas transport between adjacent cells. The positive electrode chambersand negative electrode chambers of individual redox flow battery cellsare suitably in fluid communication via common positive and negativefluid manifolds in the stack. In this way, individual electrochemicalcells can be stacked in series to yield a desired operational voltage.

Several redox flow batteries may be connected in series via electricallyconductive, preferably non-porous material which allows for electrontransport but prevents fluid or gas transport between adjacent cells(e.g., a bipolar plate) in a bipolar redox flow battery stack. Positiveand negative electrode compartments of each cell are preferablyconnected via common positive and negative fluid manifolds in the stack.Thereby, individual electrochemical cells can be stacked in series toyield a desired operational voltage.

The term “bipolar plate” refers to an electrically conductive,substantially nonporous material that may serve to separate cells in acell stack such that the cells are connected in series and the cellvoltage is additive across the cell stack. The bipolar plate typicallyhas two surfaces such that one surface of the bipolar plate serves as asubstrate for the positive electrode in one cell and the negativeelectrode in an adjacent cell. The bipolar plate typically comprisescarbon and carbon containing composite materials.

5.8 Energy Storage and Generation Systems

Redox flow batteries and redox flow battery cell stacks of the inventionmay find use in a variety of different applications, including energystorage systems for (renewable) energy, e.g. solar, wave, hydroelectricor wind energy. Further, redox flow batteries may be used as part ofpower supply (or energy generation) systems for portable devices,transport, and any other application which requires uninterruptablepower supply.

In a further aspect, the present invention thus relates to the use ofthe inventive redox flow batteries and redox flow battery cell stacksfor storing or providing electrical energy.

Redox flow batteries and redox flow battery cell stacks of the inventionmay advantageously be incorporated in larger energy storage systems.According to a further aspect of the present invention, the inventiveredox flow battery is provided in the form of a large volume energystorage cell, where the battery of the present invention preferably hasan energy storage capacity proportional to stored liquid volume. Inparticular, the battery of the present invention is in the form of anenergy storage cell having an energy storage capacity up to GW dependingon stored volume of liquids. Large volume energy storage systems areparticularly useful for the storage of wind, wave, hydroelectric orsolar energy. The energy storage systems according to the presentinvention may suitably include piping and controls useful for operationof these large units. Piping, control, and other equipment suitable forsuch systems are known in the art, and include, for example, piping andpumps in fluid communication with the respective electrolyte chambersfor moving electrolytes into and out of the respective chambers andstorage tanks for holding charged and discharged electrolytes.

The storage tanks contain the electrolytes; the tank volume determinesthe quantity of energy stored in the system, which may be measured inkWh. The control software, hardware, and optional safety systemssuitably include sensors, mitigation equipment and otherelectronic/hardware controls and safeguards to ensure safe, autonomous,and efficient operation of the flow battery energy storage system. Suchsystems are known to those of ordinary skill in the art. A powerconditioning unit may be used at the front end of the energy storagesystem to convert incoming and outgoing power to a voltage and currentthat is optimal for the energy storage system or the application. Forthe example of an energy storage system connected to an electrical grid,in a charging cycle the power conditioning unit would convert incomingAC electricity into DC electricity at an appropriate voltage and currentfor the electrochemical stack.

In a discharging cycle, the stack produces DC electrical power and thepower conditioning unit converts to AC electrical power at theappropriate voltage and frequency for grid applications.

The energy storage and generation systems described herein may alsoinclude electrolyte circulation loops, which may comprise one or morevalves, one or more pumps, and optionally a pressure equalizing line.

The energy storage and generation systems of this invention may alsoinclude an operation management system. The operation management systemmay be any suitable controller device, such as a computer ormicroprocessor, and may contain logic circuitry that sets operation ofany of the various valves, pumps, circulation loops, and the like.

The energy storage and generation systems of the present invention arepreferably suited to sustained charge or discharge cycles of severalhour durations. As such, these systems of the may be used to smoothenergy supply/demand profiles and provide a mechanism for stabilizingintermittent power generation assets (e.g., from renewable energysources). Non-limiting examples of such applications include those wheresystems of the present invention are connected to an electrical gridinclude, so as to allow renewables integration, peak load shifting, gridfirming, baseload power generation consumption, energy arbitrage,transmission and distribution asset deferral, weak grid support, and/orfrequency regulation. Redox flow battery cells, stacks, or systemsaccording to the present invention may also be used to provide stablepower for applications that are not connected to a grid, or amicro-grid.

6. Kit

In a further aspect, the present invention provides a kit comprising theinventive combination. Specifically, said kit may comprise a first redoxactive composition comprising at least one first redox active compound,and a second redox active composition comprising at least one secondredox active compound, as described herein.

The compositions may be provided in suitable containers. The first andsecond redox active composition may be provided in separated containers.The kit may optionally include further components, such as furtherdevices, software or hardware as described in the context of theinventive redox flow battery, manuals or instructions for its use.

7. Method of Storing Electrical Energy

In a further aspect, the invention provides a method of storingelectrical energy, comprising applying a potential difference across thefirst and second electrode of a redox flow battery as described herein,wherein the first redox active compound comprised by the first redoxactive composition (preferably: the posilyte), is oxidized. Preferably,the second redox active compound comprised by the second redox activecomposition (preferably: the negolyte), is reduced.

Specifically, in case the first redox active composition (preferably:posilyte) includes redox active quinone compounds according to GeneralFormula (1) or (2), preferably General Formula (1), the method mayinvolve the oxidation of said compounds, thereby generating quinones asrepresented by General Formula (1)(b) or (2)(b), preferably GeneralFormula (1)(b). The respective oxidation reaction is illustrated byReaction Schemes (I) and (II) above.

Furthermore, in case the second redox active composition (preferably:negolyte) includes redox active quinone compounds according to GeneralFormula (2) or (3), preferably General Formula (3), the method mayinvolve the reduction of said compounds, thereby generatinghydroquinones as represented by General Formulas (2)(a) and (3)(a),preferably General Formula (3)(a). The respective reduction reaction isillustrated by Reaction Schemes (II) and (III) above.

8. Method of Providing Electrical Energy

In a further aspect, the present invention relates to a method ofproviding electrical energy, comprising applying a potential differenceacross the first and second electrode of a redox flow battery asdescribed herein, wherein the first redox active compound comprised bythe first redox active composition (preferably: the posilyte), isreduced. Preferably, the second redox active compound comprised by thesecond redox active composition (preferably: the negolyte), is oxidized.

Specifically, in case the first redox active composition (preferably:posilyte) includes redox active quinone compounds according to GeneralFormula (1) or (2), preferably General Formula (1), the method mayinvolve the reduction of said compounds, thereby regeneratinghydroquinones as represented by General Formula (1)(a) or (2)(a),preferably General Formula (1)(a). The respective reduction reaction isillustrated by Reaction Schemes (I) and (II) above.

Furthermore, in case the second redox active composition (preferably:negolyte) includes redox active quinone compounds according to GeneralFormula (2) or (3), preferably General Formula (3), the method mayinvolve the oxidation of said compounds, thereby regenerating thequinones as represented by General Formulas (2)(b) or (3)(b), preferablyGeneral Formula (3)(b), is preferably formed. The respective oxidationreaction is illustrated by Reaction Schemes (II) and (III) above.

9. DESCRIPTION OF THE FIGURES

FIG. 1 schematically illustrates a redox flow battery comprising theinventive combination of redox active compositions and its function.

FIG. 2 A-K shows charge/discharge curves of selected redox activequinone compounds on graphite electrodes.

FIG. 3 shows further preferred compounds according to General Formulas(1), (2) and (3).

10. EXAMPLES

In the following, particular examples illustrating various embodimentsand aspects of the invention are presented. However, the presentinvention shall not to be limited in scope by the specific embodimentsdescribed herein. The following preparations and examples are given toenable those skilled in the art to more clearly understand and topractice the present invention. The present invention, however, is notlimited in scope by the exemplified embodiments, which are intended asillustrations of single aspects of the invention only, and methods whichare functionally equivalent are within the scope of the invention.Indeed, various modifications of the invention in addition to thosedescribed herein will become readily apparent to those skilled in theart from the foregoing description, accompanying figures and theexamples below. All such modifications fall within the scope of theappended claims.

Example 1: Preparation of Low Molecular Weight Aromatic Lignin-DerivedCompounds by Cracking and Reduction by a Nickel Catalyst

Reductive cracking of a modified lignin-derived component was carriedout by means of a catalyst comprising nickel, e.g. supported onactivated carbon (Ni/C). The catalysts are typically prepared by anincipient-wetness impregnation method and further treated by acarbothermal reduction method known in the art.

Herein, nickel nitrate(II) hexahydrate [Ni(NO₃)₂ 6H₂O] is used andoptionally added into water in a beaker known in the art. The solutionis then stirred, e.g. for at least 30 min, to prepare an impregnationstock solution. Activated carbon having a water absorption capacity oftypically above 1.8 mL g⁻¹ is added into the solution and the beaker maythen covered by a culture dish to keep the sample wet for a prescribedtime, preferably more than 12 h, more preferably 24 h. The sample isthen dried at a temperature above 80° C., e.g. 120° C. overnight. Theactual reduction is carried out in a container such as a preferablyhorizontal furnace in a flow of inert gas such as N₂. The flow is, e.g.,10 mL min⁻¹ or more, preferably 30 mL min⁻¹ or more. The reductiontemperature preferably reaches at least 400° C., preferably 450° C.,e.g. over set time period such as at least 30 min, preferably at least60 min. The temperature for conducting the reduction is maintained at450° C. for at least 1 h, more preferably for at least 2 h. TheNi/SBA-15 catalysts are reduced at 550° C. for 2 h. The Ni/Al₂O₃catalyst is reduced at 700° C. for 2 h. The metal loading for eachnickel- and copper-based catalyst is 10% (w/w) relative to the support.Herein, birch sawdust serves as lignocellulosic material and is treatedwith the ethanol-benzene mixture (v/v ratio 1:2) for 12 h. The treatedbirch sawdust, solvent (m/v 1:20), and catalyst (w/w 20:1) are placed inan autoclave reactor. The reactor is sealed and purged with Ar 4 to 6times to expel air. Then, the reducing reaction is conducted at 200° C.at a stirring speed of at least 300 rpm, preferably 500 rpm. When thedesired reaction time (usually 2 to 10 h) is reached, the reactor iscooled to ambient temperature before sampling.

Typically, the reaction generates 4-propylguaiacol and 4-propylsyringolas major products, together with minor alkene-substituted4-propylguaiacol and 4-propylsyringol, as determined by standard gaschromatography. The compounds are isolated according to step (F),preferably by extraction.

Example 2: Preparation of Monomeric Aromatic Lignin-Derived Moleculesfrom Lignosulfonate of a Sulfite Process by Electrooxidation

A 1 M aqueous NaOH solution of lignosulfonate is prepared, comprising 1%(W/W) lignosulfonate. Said solution is subjected to an electrooxidation.Therein, the solution is employed as anolyte. A 1 M aqueous solution isemployed as catalyte. A flow cell with a flow rate of 250 ml/min isused. Electrolysis is allowed to take place galvanostatically for 8 happlying current of 1 mA/cm². A typical resulting voltage is 1,4 V. Thevoltage curve typically is asymptotic and the solution changespreferably color from brown to dark brown.

Samples of the solution are taken every hour over a time span of 8 h andsubsequently examined photometrically. Thereof, an absorption profiletypical for ortho-benzoquinone is determined. Hence, a lower molecularweight aromatic lignin-derived compound, quinone compound, is preparedby said method.

Said compound is then isolated. Therefore, said compound is extracted bydichloromethane and subsequently subjected to cycles of charging anddischarging processes in a flow cell. The voltage curve shows that thecompound is redox active, which may be reversibly electrolyzed.

Example 3: Preparation of an Annulated Quinone Compound by aFriedel-Crafts Acylation

Vanillin as a low molecular weight aromatic lignin-derived compound isprovided and further annulated and oxidized in five steps as follows:

(i) Synthesis of 4-(benzyloxy)-3-methoxybenzaldehyde (2)

Vanillin (1) (1.0 eq.) and benzyl chloride (1.2 eq.) are dissolved inN,N-dimethylformamide and potassium iodine (0.5 mol %) is added.Afterwards potassium carbonate is added and the reaction is stirredabove 60° C., preferably between 60 to 120° C. for at least 1 h,preferably 1 to 8 h. After completion of the reaction, the solution isdiluted with distilled water and extracted with an appropriate solvent.The organic phase is washed with brine and the product is then isolatedfrom the organic phase.

(ii) Synthesis of 4-(benzyloxy)-3-methoxybenzoic acid (3)

A mixture of 1,2-dimethoxyethane and potassium hydroxide (5 to 20 eq.)is purged with oxygen and the calculated amount of isolated product 2(1.0 eq.) is added. After the absorption of oxygen ceases, the mixtureis diluted with distilled water and neutral organic products areextracted with an appropriate solvent. The aqueous layer is acidifiedand the acidic organic products are extracted with an appropriatesolvent. Product 3 is isolated from the organic layer.

(iii) Synthesis of 4-(benzyloxy)-3-methoxybenzoyl chloride (4)

Isolated product 3 (1.0 eq.) is dissolved in thionyl chloride (5-20 eq.)and the mixture is stirred at 60 to 120° C. for 1 to 8 h. Aftercompletion of the reaction excess thionyl chloride is evaporated toyield desired acyl chloride 4.

(iv) Synthesis of anthraquinones (5-7)

Aluminiumtrichloride (0.1 eq.) is added to the crude acyl chloride 4 andthe mixture is stirred for 30 to 300 min at −20 to 60° C. Aftercompletion of the reaction the mixture is carefully quenched with bicarbsolution. The product is extracted with an appropriate solvent and theorganic layer is washed with brine. The product is then isolated fromthe organic phase.

(v) Synthesis of 2,6-dihydroxy-3,7-dimethoxyanthracene-9,10-dione 8 and2,6-dihydroxy-1,7-dimethoxyanthracene-9,10-dione 9

Anthraquinone 5 or 6 are dissolved in ethyl acetate, methanol or ethanoland palladium on charcoal (1 to 30 weight %) is added. The mixture isstirred at room temperature under hydrogen atmosphere (1-10 bar). Thecatalyst is filtered off and the product (9) is isolated from themixture.

The product is then characterized by spectrographic means, and providedas redox active compound according to the present invention.

Example 4: Derivatization of (Hydro-)Quinones

Substituents are introduced into the low molecular weight lignin-derivedcomponents.

Example 4.1 Reduction of Dimethoxy Benzoquinone

23.2 g of sodium dithionite (0.134 mol, 1.32 eq.) was added to thesuspension of 17.0 g (0.101 mol, 1.0 eq.)2,6-dimethoxycyclohexa-2,5-diene-1,4-dione in 100 mL H₂O. After 2 hstirring at room temperature the precipitate was filtered off and driedin the air to give 15.85 g (0.093 mol, 92% yield) of2,6-dimethoxybenzene-1,4-diol as a white solid.

Example 4.2: Oxidation of Methoxy Benzohydroquinone

1.4 g of catalyst Cu/AlO(OH) was added to a solution of 8.2 g (0.059mol) 2-methoxy-1,4-dihydroxybenzene in 250 ml ethyl acetate, and thereaction mixture was stirred at room temperature for 147 h under an O₂atmosphere. After the conversion determined by HPLC reached 99%, thereaction mixture was filtered, and the recovered catalyst was washedwith ethyl acetate (100 mL×3). The filtrate was collected and solventwas removed in vacuo to give 7.66 g (0.055 mol, 95% yield) of2-methoxycyclohexa-2,5-diene-1,4-dione as a yellow-brownish solid.

Example 4.3: Acetylation of Methoxy Benzohydroquinone

8.24 g (0.059 mol, 1.0 eq.) of 2-methoxybenzene-1,4-diol was weighedinto a 250 mL reaction flask equipped with a reflux condenser. 60 mL ofdichloroethane and 15 mL (0.159 mol, 2.7 eq.) of acetic anhydride wereadded. 12 mL (0.096 mol, 1.63 eq.) of boron trifluoride ether solutionwas then slowly added at room temperature with stirring. The reactionmixture was heated to 90° C. for 20 hours. The mixture was cooled to 60°C., 30 mL H₂O was added followed by 10 mL HCl (6 M). The resultingmixture was heated to 100° C. for 30 min, cooled down and extracted withethyl acetate (150 mL×3). The combined extracts were washed sequentiallywith H₂O (100 mL), saturated sodium bicarbonate (100 mL) and H2O (100mL) and then dried with anhydrous sodium sulfate. The solvent wasremoved in vacuo to give a brown solid residue, which was washed withmethanol to give 7.49 g (0.041 mol, 70% yield) of1-(2,5-dihydroxy-4-methoxyphenyl)ethan-1-one as a beige solid.

Example 4.4 Addition of Isonicotinic Acid to Benzoquinone

2.16 g (0.02 mol, 1.0 eq.) of p-benzoquinone was suspended in 6.4 mL ofacetic acid. 2.46 g (0.02 mol, 1.0 eq.) of nicotinic acid was added andthe mixture was stirred for 2 h at rt. The resulting dark mixture wasdiluted with 3 mL of water and treated with 6.6 mL of HCl (6 M). Oncooling, solid precipitated which was filtered off and dried overnightat 60° C. to give 3.13 g (0.012 mol, 59% yield) of3-carboxy-1-(2,5-dihydroxyphenyl)pyridin-1-ium chloride as an yellowsolid.

Example 4.5 Sulfonation of Anthraquinone

A solution of anthraquinone was heated (180° C.) in a solution of20%-40% SO₃ in concentrated sulfuric acid (oleum), resulting in amixture of sulfonated anthraquinones. The crude mixture was poured ontoice and partially neutralized with calcium hydroxide. Subsequently, themixture was filtrated and concentrated to yield the final product.

Example 4.6: Sulfonation of hydroquinone (1,4-Dihydroxybenzene)

A solution of hydroquinone was heated (80° C.) in a solution of 20%-40%SO₃ in concentrated sulfuric acid (oleum), resulting in a mixture ofsulfonated hydroquinones. The crude mixture was poured onto ice andpartially neutralized with calcium hydroxide. Subsequently, the mixturewas filtrated and concentrated to yield the final product.

Example 4.7: Sulfonation of 1,4-Dihydroxy-2,6-dimethoxybenzene

A solution of hydroquinone was heated (80° C.) in a solution of 20%-35%SO₃ in concentrated

sulfuric acid (oleum), resulting in a mixture of sulfonated1,4-dihydroxy-2,6-dimethoxybenzenes. The crude mixture was poured ontoice and partially neutralized with calcium hydroxide. Subsequently, themixture was filtrated and concentrated to yield the final product.

Example 4.8: Sulfonation of 2-Methoxyhydroquinone

A solution of 2-methoxyhydroquinone was heated (80° C.) in a solution of20%-40% SO₃ in concentrated sulfuric acid (oleum), resulting in amixture of sulfonated 2-methoxyhydroquinones. The crude mixture waspoured onto ice and partially neutralized with calcium hydroxide.Subsequently, the mixture was filtrated and concentrated to yield thefinal product.

Example 4.9: Synthesis of2,5-bis{[(2-hydroxyethyl)(methyl)amino]methyl}benzene-1,4-diol

In a round-bottom flask 40.0 g hydroquinone (0.36 mol, 1 eq) and 24.0 gparaformaldehyde (0.80 mol, 2.2 eq) were dissolved in toluene (200 mL).64 mL 2-(methylamino)ethanol (0.80 mol, 2.2 eq) was added and thereaction mixture was heated under reflux for 20 h. After cooling to roomtemperature the solvent was removed in vacuum and the residue wasrecrystallized from acetone to yield 65.2 g of product (63% yield) as anoff-white solid.

Example 4.10: Synthesis of2,6-bis[(dimethylamino)methyl]-3,5-dimethoxybenzene-1,4-diol

8.51 g 2,6-dimethoxyhydroquinone (50 mmol, 1 eq) and 3.30 gparaformaldehyde (110 mmol, 2.2 eq) were dissolved in ethanol (130 mL).19 mL of dimethylamine solution in ethanol (5.6 M, 110 mmol, 2.2 eq) wasadded and the reaction mixture was stirred at room temperature for 20 h.After completion of the reaction, the solvent was removed in vacuum toobtain 12.2 g of product (86% yield). Analytically pure sample wasobtained by recrystallization from acetone.

Example 5: Cycling Tests of Different Combinations of Redox ActiveQuinone Compounds

Cycling Tests

Selected redox active compositions were subjected to electrochemicalmeasurements. Therefore, a small laboratory cell employing selectedquinones/hydroquinones in different combinations as positive andnegative redox active compounds were evaluated with constant-currentcharge-discharge experiments and open-circuit voltage measurement with aBaSyTec battery test system (BaSyTec GmbH, 89176 Asselfingen, Germany).This cell consists of four main parts: a graphite felt (with an area of6 cm², 6 mm in thickness) was employed as the positive and negativeelectrode, and a cation exchange membrane was used to separate thepositive and negative electrolytes. Positive and negative electrolytesevaluated in each cycling test are specified in table 5 below. Theelectrolytes were provided in an aqueous solution of 20% sulfuric acidin water. The pH of the electrolyte solutions was <0. No additives wereused. The electrolytes were pumped by peristaltic pumps to thecorresponding electrodes, respectively. In the charge-discharge cycles,the cell was charged at a current density of 10 mA cm² up to 1.2 V anddischarged at the same current density down to −0.4 V cut-off.

TABLE 5 Evaluated electrolyte combinations Open Circuit CoulombicVoltage efficiency # Posilyte Negolyte (OCV) (CE) Figure 1

0.69 V 97% 2A 2

0.79 V 93% 2B 3

0.70 V 97% 2C

4

0.64 V 98% 2D 5

0.72 V 97% 2E 6

0.66 V 99% 2F 7

0.71 V 91% 2G 8

0.68 V 90% 2H 9

0.75 V 99.5%  21

10

0.73 V 96% 21

11

0.68 V 96% 2K

FIG. 2A-K show the charge/discharge curves of selected redox activequinone compounds on graphite electrodes.

1. A combination of: (1) a first redox active composition comprising afirst redox active compound, the first redox active compound beingcharacterized by any of general formulas (1)-(3), more preferablygeneral formulas (1) or (2) and most preferably general formula (1), ormixtures thereof; and (2) a second redox active composition comprising asecond redox active compound, the second redox active compound beingcharacterized by any of general formulas (1)-(3), more preferablygeneral formulas (2) or (3), and most preferably by general formula (3),or mixtures thereof:

wherein R¹-R¹⁸ is each independently selected from hydrogen; hydroxyl;carboxy; linear or branched, optionally substituted C₁₋₆ alkyloptionally comprising at least one heteroatom selected from N, O and S,including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂ and —C_(n)H_(2n)SO₃H, whereinn is an integer selected from 1, 2, 3, 4, 5, or 6; carboxylic acids;esters; halogen; optionally substituted C₁₋₆ alkoxy, including methoxyand ethoxy; optionally substituted amino, including primary, secondary,tertiary and quaternary amines, in particular —NH₂/NH₃ ⁺, —NHR/NH₂R⁺,—NR₂/NHR₂ ⁺ and —NR₃ ⁺, where R is H or optionally substituted C₆ alkyloptionally comprising at least one heteroatom selected from N, O and S,including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, —C_(n)H_(2n)NR₂,—C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H, wherein n is an integer selectedfrom 1, 2, 3, 4, 5, or 6, where R is H or optionally substituted C₁₋₆alkyl optionally comprising at least one heteroatom selected from N, Oand S, including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂, C_(n)H_(2n)CO₂H and—C_(n)H_(2n)SO₃H; amide; nitro; carbonyl; phosphoryl; phosphonyl;cyanide; and sulfonyl (—SO₃H), wherein preferably at least one of R¹-R⁴in General Formula (1), at least one of R⁵-R¹⁰ in General Formula (2)and/or at least one of R¹¹-R¹⁸ in General Formula (3) is selected from—SO₃H; optionally substituted C₁₋₆ alkyl optionally comprising at leastone heteroatom selected from N, O and S, including —C_(n)H_(2n)OH,—C_(n)H_(2n)NH₂ and —C_(n)H_(2n)SO₃H, wherein n is an integer selectedfrom 1, 2, 3, 4, 5, or 6; optionally substituted amino, includingprimary, secondary, tertiary and quaternary amines, in particular—NH₂/NH₃ ⁺, —NHR/NH₂R⁺, —NR₂/NHR₂ and —NR₃ ⁺, where R is H or optionallysubstituted C₁₋₆ alkyl optionally comprising at least one heteroatomselected from N, O and S, including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂,—C_(n)H_(2n)NR₂, —C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H, wherein n is aninteger selected from 1, 2, 3, 4, 5, or 6, where R is H or optionallysubstituted C₁₋₆ alkyl optionally comprising at least one heteroatomselected from N, O and S, including —C_(n)H_(2n)OH, —C_(n)H_(2n)NH₂,C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H; and optionally substituted C₁₋₆alkoxy, preferably methoxy.
 2. The combination according to claim 1,wherein the first and second redox active compound are characterized bydifferent General Formulas (1)-(3).
 3. The combination according toclaim 1 or 2, wherein said first and second redox active compound arewater-soluble.
 4. The combination according to any one of claims 1 to 3,wherein said first and second redox active composition are liquid. 5.The combination according to any one of the preceding claims, whereinthe first and second redox active composition are provided in separatecompartments.
 6. The combination according to claim 5, wherein the firstand second redox active composition are each provided in a half-cell ofa redox flow battery.
 7. The combination according to any one of thepreceding claims, wherein the first redox active composition comprisesat least one first redox active compound as characterized by any ofGeneral Formulas (1), (2) or (3), preferably General Formulas (1) or(2), and most preferably General Formula (1), or mixtures thereof;optionally including at least one reduction and/or oxidation productthereof as characterized by General Formula (1)(a) or (b), (2)(a) or(b), or (3)(a) or (b); or mixtures thereof.
 8. The combination accordingto any one of the preceding claims, wherein the second redox activecomposition comprises at least one second redox active compoundcharacterized any of General Formulas (1), (2) or (3), preferablyGeneral Formulas (2) or (3), and most preferably General Formula (3), ormixtures thereof; optionally including at least one reduction and/oroxidation product thereof as characterized by General Formula (1)(a) or(b), (2)(a) or (b), or (3)(a) or (b); or mixtures thereof.
 9. Thecombination according to any one of the preceding claims, wherein thefirst redox active composition comprises as a first redox activecompound at least one benzohydroquinone characterized by General Formula(1), optionally including at least one reduction and/or oxidationproduct thereof as characterized by General Formula (1)(a) or (b); ormixtures thereof.
 10. The combination according to any one of thepreceding claims, wherein the second redox active composition comprisespreferably as second redox active compound at least one anthraquinonecharacterized by General formula (3), optionally including at least onereduction and/or oxidation product thereof as characterized by Generalformula (3) (a) or (b); or preferably as a second redox active compoundat least one benzohydroquinone characterized by General Formula (1),optionally including at least one reduction and/or oxidation productthereof as characterized by General Formula (1)(a) or (b); or mixturesthereof; or as a second redox active compound at least onenaphthoquinone characterized by General formula (2), optionallyincluding at least one reduction and/or oxidation product thereof ascharacterized by General formula (2)(a) or (b); or mixtures thereof. 11.The combination according to any one of the preceding claims, wherein inGeneral Formula (1): R¹ is selected from —H, —SO₃H, optionallysubstituted C₁₋₆ alkyl and optionally substituted amine; R² is selectedfrom —H, —OH, —SO₃H, C₁₋₆ alkoxy, preferably methoxy, and optionallysubstituted amine; R³ is selected from —H, —OH and C₁₋₆ alkoxy,preferably methoxy; and R⁴ is selected from —H, —SO₃H, optionallysubstituted C₁₋₆ alkyl, optionally substituted amine and halogen. 12.The combination according to claim 11, wherein R¹ and/or R⁴ areindependently selected from substituted C₁₋₆ alkyl selected from R⁵—SO₃Hand R⁵—CO₂H, wherein R⁵ is a C₁₋₆ alkyl optionally comprising at leastone heteroatom selected from N, O or S.
 13. The combination according toclaim 11, wherein R¹, R² and/or R⁴ are independently selected from—NH₂/NH₃ ⁺, —NHR/NH₂R⁺, —NR₂/NHR₂ ⁺ and —NR₃ ⁺, where R is H oroptionally substituted C₁₋₆ alkyl, optionally comprising at least oneheteroatom selected from N, O and S, including —C_(n)H_(2n)OH,—C_(n)H_(2n)NH₂, —C_(n)H_(2n)NR₂, —C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H,wherein n is an integer selected from 1, 2, 3, 4, 5, or 6, where R is Hor optionally substituted C₁₋₆ alkyl optionally comprising at least oneheteroatom selected from N, O and S, including —C_(n)H_(2n)OH,—C_(n)H_(2n)NH₂, C_(n)H_(2n)CO₂H and —C_(n)H_(2n)SO₃H.
 14. Thecombination according to claim 13, wherein the compounds of GeneralFormula (1) are characterized by one of formulas (1.1)-(1.10):

or a quinone form thereof.
 15. The combination according to any one ofthe preceding claims, wherein in General Formula (2): R⁵ and R⁶ areindependently selected from —H, —OH and C₁₋₆ alkoxy, preferably methoxy;and R¹-R¹⁰ are independently selected from —H and —SO₃H.
 16. Thecombination according to any one of the preceding claims, wherein inGeneral Formula (3): R¹¹, R¹² and R¹⁴ are independently selected from—H, —OH and C₁₋₆ alkoxy, preferably methoxy; and R¹³ and R¹⁵-R¹⁸ areindependently selected from —H and —SO₃H.
 17. The combination accordingto any one of the preceding claims, wherein in General Formula (3): R¹¹is —SO₃H; R¹² is —SO₃H, R¹¹, R¹³ and R¹⁴ are preferably —OH; R¹⁶ is—SO₃H, R¹¹ and R¹⁴ or R¹¹, R¹² and R¹⁴ are preferably —OH; R¹² and R¹⁶are —SO₃H, R¹¹ and R¹⁴ or R¹¹, R¹³ and R¹⁴ are preferably —OH; R¹³ andR¹⁶ are —SO₃H, R¹¹, R¹² and R¹⁴ are preferably —OH; R¹² and R¹⁷ are—SO₃H; or R¹¹ and R¹⁴ are —SO₃H; wherein each of the others of R¹¹-R¹⁸is/are C₁₋₆ alkoxy or —H, preferably —H.
 18. The combination accordingto claim 17, wherein the compounds of General Formula (3) arecharacterized by formula (6.1), or a hydroquinone form thereof:


19. The combination according to any one of claims 12 to 18, comprising(1) a first redox active compound selected from: at least onebenzohydroquinone characterized by formula (1.1)-(1.6) or (1.9):

or mixtures thereof, and optionally oxidation products thereof; and (2)a second redox active compound selected from: preferably at least oneanthraquinone characterized by formula (6.1)

and optionally reduction products thereof; or at least onebenzohydroquinone characterized by formula (1.7) or (1.8) or (1.10):

or mixtures thereof, and optionally oxidation products thereof; or 20.The combination according any one of the preceding claims, wherein thefirst redox active composition and/or the second redox activecomposition is a liquid.
 21. The combination according to any one of thepreceding claims, wherein the first and/or the second redox activecomposition further comprises a solvent, optionally selected from water,ionic liquids, methanol, ethanol, propanol, isopropanol, acetonitrile,acetone, dimethylsulfoxide, glycol, carbonates such aspropylenecarbonate, ethylene carbonate, propylene carbonate, dimethylcarbonate, diethyl carbonate, methyl ethyl carbonate, propylenecarbonate; polyethers such as dimethoxyethane, γ-butyrolactone;tetrahydrofuran; dioxolane; sulfolane; dimethylformamide;diethylformamide; CO₂ and supercritical CO₂; or a mixture thereof. 22.The combination according to claim 21, wherein the solvent comprises atleast at least about 40%, at least about 50 wt %, at least about 60 wt%, at least about 70 wt %, at least about 75 wt %, at least about 80%,at least about 85 wt %, at least about 90 wt %, at least about 95 wt %,or at least about 98 wt % water, relative to the total solvent.
 23. Thecombination according to any one of the preceding claims, wherein thefirst and/or the second redox active composition further comprises anadditive selected from co-solvents; salts; buffering agents; emulsifyingagents; further redox active compounds; supporting electrolytes; ionicliquids; acids; bases; viscosity modifiers; wetting agents; stabilizers;or combinations thereof.
 24. The combination according to any one of thepreceding claims, wherein the first and/or the second redox activecomposition has a pH between <0 and about 14, between about 0 and about14, between about 7 and about 14, more preferably between about 9 toabout 14, most preferably between about 10 and about 12, or betweenabout 12 and about
 14. 25. The combination according to any one of thepreceding claims, wherein the first and/or the second redox activecompound are present in a concentration of between about 0.3 M and about12 M, preferably at least about 0.3 M, at least about 0.5 M, at leastabout 1 M, at least about 2 M, at least about 4 M, or at least about 6M, in particular between about 0.5 M and about 2 M, between about 2 Mand about 4 M, between about 4 M and about 6 M, or between about 6 M andabout 10 M.
 26. The combination according to any one of the precedingclaims, wherein the first redox active compound has a standard reductionpotential that is at least 0.3 volts higher than the standard reductionpotential of the second redox active compound.
 27. The combinationaccording to claim any one of the preceding claims, wherein the firstredox active compound has a standard reduction potential of at leastabout 0.0 volts, preferably of at least about +0.5 V, even morepreferably of at least about +0.6 V, most preferably of at least about+0.7 V or more against a standard hydrogen electrode; and/or wherein thesecond redox active compound has a standard electrode potential of about+0.3 V or less, more preferably of about +0.1 V or less, even morepreferably of about 0.0 V or less, about −0.5 V or less, about −0.6V orless, about −1.0V or less or about −1.2 V or less against a standardhydrogen electrode.
 28. A kit comprising the combination according toany one of the preceding claims, wherein the first and second redoxactive composition are preferably provided in separate containers. 29.Use of a combination according to any one of the preceding claims or akit according to claim 23 for preparing redox flow battery electrolytes,wherein the first redox active composition is preferably used as apositive electrode electrolyte, and the second redox active compositionis preferably used as a negative electrode electrolyte.
 30. A redox flowbattery comprising: a positive electrode; a first redox activecomposition according to any one of claims 1 to 27 as a positiveelectrode electrolyte, the positive electrode electrolyte contacting thepositive electrode; a negative electrode; a second redox activecomposition according to any one of claims 1 to 27 as a negativeelectrode electrolyte, the negative electrode electrolyte contacting thenegative electrode; and a separator interposed between the positiveelectrode and the negative electrode.
 31. The redox flow batteryaccording to claim 30, further comprising: a positive electrodereservoir comprising the positive electrode immersed within the positiveelectrode electrolyte, said positive electrode reservoir forming thefirst redox flow battery half-cell; and a negative electrode reservoircomprising the negative electrode immersed within the negative electrodeelectrolyte, said negative electrode reservoir forming the second redoxflow battery half-cell.
 32. The redox flow battery according to claim31, wherein said redox flow battery is charged by applying a potentialdifference across the first and second electrode, such that the firstredox active compound is oxidized and the second redox active compoundis reduced.
 33. The redox flow battery according to claim 32, whereinthe redox flow battery is discharged by applying a potential differenceacross the first and second electrode such that the first redox activecompound is reduced, and the second redox active compound comprised bysaid composition is oxidized.
 34. The redox flow battery according toany one of claims 30 to 33, wherein the separator comprises oressentially consists of a cation exchange membrane, optionally selectedfrom a polymer membrane, more preferably from a sulfonate containingfluoropolymer.
 35. The redox flow battery according to any one of claims30 to 34, wherein the positive and negative electrode comprise oressentially consist of a metal, a carbon material or anelectro-conductive polymer.
 36. The redox flow battery according to anyone of claims 30 to 35, further comprising: a first circulation loopcomprising a storage tank containing the positive electrode electrolyte,piping for transporting the positive electrode electrolyte, a chamber inwhich the first electrode is in contact with the positive electrodeelectrolyte, and a pump to circulate the positive electrode electrolytethrough the circulation loop; optionally a second circulation loopcomprising a storage tank containing the negative electrode electrolyte,piping for transporting the negative electrode electrolyte, a chamber inwhich the second electrode is in contact with the negative electrodeelectrolyte, and a pump to circulate the negative electrode electrolytethrough the circulation loop; and optionally control hardware andsoftware.
 37. A redox flow battery cell stack comprising at least tworedox flow batteries according to any one of claims 30 to
 36. 38. Use ofa redox flow battery or redox flow battery cell stack according to anyone of claims 30 to 37 for storing or providing electrical energy. 39.An energy storage system comprising a redox flow battery or redox flowbattery cell stack according to any one of claims 30 to 37; connected toan electrical grid.
 40. A method of storing electrical energy,comprising applying a potential difference across the first and secondelectrode of a redox flow battery according to claim any one of claims30 to 37, wherein the first redox active compound is oxidized.
 41. Themethod according to claim 40, wherein the second redox active compoundcomprised by the second redox active composition is reduced.
 42. Amethod of providing electrical energy, comprising applying a potentialdifference across the first and second electrode of a redox flow batteryaccording to any one of claims 30 to 37, wherein the first redox activecompound is reduced.
 43. The method according to claim 42, wherein thesecond redox active compound is oxidized.